nrf52820_microbit: Start HC xtal & select RC LF clock.

nrfx_clock_enable() was removed as it basically enables the clock
interrupts which we don't need.

Author: microbit-carlos

source/hic_hal/nordic/nrf52820/nrfx/nrfx_config_nrf52820.h

@@ -56,7 +56,7 @@
 // <196609=> External Full Swing
 
 #ifndef NRFX_CLOCK_CONFIG_LF_SRC
-#define NRFX_CLOCK_CONFIG_LF_SRC 1
+#define NRFX_CLOCK_CONFIG_LF_SRC 0
 #endif
 
 // <q> NRFX_CLOCK_CONFIG_LF_CAL_ENABLED  - Enables LF Clock Calibration Support

source/hic_hal/nordic/nrf52820/sdk.c

@@ -82,5 +82,7 @@ void sdk_init()
     }
 #endif
 
-    nrfx_clock_enable();
+    if (!nrfx_clock_hfclk_is_running()) {
+        nrfx_clock_hfclk_start();
+    }
 }