[NORDIC - NRF51 - MBED 2] Fix a bug related to the RTC interrupt enabled.

Enable the interrupt for the OS tick when the OS tick is enabled rather than
all the time. Otherwise, the interrupt will be triggered bu never handled.

Author: pan-

targets/TARGET_NORDIC/TARGET_NRF5/us_ticker.c

@@ -110,9 +110,6 @@ void common_rtc_init(void)
     // events will be enabled or disabled as needed (such approach is more
     // energy efficient).
     nrf_rtc_int_enable(COMMON_RTC_INSTANCE,
-    #if defined(TARGET_MCU_NRF51822)
-        OS_TICK_INT_MASK |
-    #endif
     #if DEVICE_LOWPOWERTIMER
         LP_TICKER_INT_MASK |
     #endif
@@ -506,6 +503,7 @@ static void register_next_tick() {
 int os_tick_init (void)
 {
     common_rtc_init();
+    nrf_rtc_int_enable(COMMON_RTC_INSTANCE, OS_TICK_INT_MASK);
 
     nrf_rtc_cc_set(COMMON_RTC_INSTANCE, OS_TICK_CC_CHANNEL, 0);
     register_next_tick();