Re-implement RTC for Silicon Labs targets

mbed RTC specifications now dictate that the RTC needs to retain and keep on counting through reset. On Silicon Labs parts, this means the RTC API can not be backed by the Silicon Labs RTC peripheral, since that doesn't provide retention functionality.
Therefore:
* On EFM32GG, EFM32WG, EFM32LG: mbed RTC API is now backed by BURTC.
* On EFM32PG, EFR32MG, EFM32PG12, EFR32MG12: mbed RTC API is now backed by RTCC.
* On EFM32ZG, EFM32HG: mbed RTC API is sadly no longer supported, since these chips don't have retained memory.

Author: stevew817

targets/TARGET_Silicon_Labs/TARGET_EFM32/lp_ticker.c

@@ -125,6 +125,20 @@ void lp_ticker_free()
         CMU_ClockEnable(cmuClock_RTC, false);
         rtc_inited = false;
     }
+    RTC_FreezeEnable(false);
+}
+
+void rtc_set_comp0_value(uint64_t value, bool enable)
+{
+    rtc_enable_comp0(false);
+
+    /* Set callback */
+    RTC_FreezeEnable(true);
+    extended_comp0 = (uint32_t) (value >> RTC_BITS);
+    RTC_CompareSet(0, (uint32_t) (value & RTC_MAX_VALUE));
+    RTC_FreezeEnable(false);
+
+    rtc_enable_comp0(enable);
 }
 
 void lp_ticker_set_interrupt(timestamp_t timestamp)

targets/targets.json

@@ -2919,7 +2919,7 @@
     "EFM32GG_STK3700": {
         "inherits": ["EFM32GG990F1024"],
         "progen": {"target": "efm32gg-stk"},
-        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH", "ITM"],
+        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH", "ITM"],
         "forced_reset_timeout": 2,
         "config": {
             "hf_clock_src": {
@@ -2972,7 +2972,7 @@
     },
     "EFM32LG_STK3600": {
         "inherits": ["EFM32LG990F256"],
-        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
+        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
         "forced_reset_timeout": 2,
         "device_name": "EFM32LG990F256",
         "config": {
@@ -3027,7 +3027,7 @@
     "EFM32WG_STK3800": {
         "inherits": ["EFM32WG990F256"],
         "progen": {"target": "efm32wg-stk"},
-        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
+        "device_has": ["ANALOGIN", "ANALOGOUT", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
         "forced_reset_timeout": 2,
         "config": {
             "hf_clock_src": {
@@ -3188,7 +3188,7 @@
     },
     "EFM32PG_STK3401": {
         "inherits": ["EFM32PG1B100F256GM32"],
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
         "forced_reset_timeout": 2,
         "config": {
             "hf_clock_src": {
@@ -3251,7 +3251,7 @@
     },
     "EFR32MG1_BRD4150": {
         "inherits": ["EFR32MG1P132F256GM48"],
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
         "forced_reset_timeout": 2,
         "config": {
             "hf_clock_src": {
@@ -3294,7 +3294,7 @@
     },
     "TB_SENSE_1": {
         "inherits": ["EFR32MG1P233F256GM48"],
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "FLASH"],
         "forced_reset_timeout": 5,
         "config": {
             "hf_clock_src": {
@@ -3342,7 +3342,7 @@
     },
     "EFM32PG12_STK3402": {
         "inherits": ["EFM32PG12B500F1024GL125"],
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "TRNG", "FLASH"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "TRNG", "FLASH"],
         "forced_reset_timeout": 2,
         "config": {
             "hf_clock_src": {
@@ -3396,7 +3396,7 @@
     "TB_SENSE_12": {
         "inherits": ["EFR32MG12P332F1024GL125"],
         "device_name": "EFR32MG12P332F1024GL125",
-        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "TRNG", "FLASH"],
+        "device_has": ["ANALOGIN", "I2C", "I2CSLAVE", "I2C_ASYNCH", "INTERRUPTIN", "LPTICKER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPISLAVE", "SPI_ASYNCH", "STDIO_MESSAGES", "USTICKER", "TRNG", "FLASH"],
         "forced_reset_timeout": 5,
         "config": {
             "hf_clock_src": {