Fix a bug preventing LEUART from being clocked correctly

Author: stevew817

libraries/mbed/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/gpio_object.h

@@ -17,6 +17,7 @@
 #define MBED_GPIO_OBJECT_H
 
 #include "em_gpio.h"
+#include "PinNames.h"
 
 #ifdef __cplusplus
 extern "C" {

libraries/mbed/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/mbed_overrides.c

@@ -50,7 +50,12 @@ void mbed_sdk_init()
     CMU_ClockSelectSet(cmuClock_LFA, LFXO);
 #endif
 #ifdef CMU_LFBCLKSEL_REG
-    CMU_ClockSelectSet(cmuClock_LFB, LFXO);
+	  /* cmuClock_LFB (to date) only has LEUART peripherals.
+		*  Do NOT set it up here, as LEUARTs might have been initialized
+		*	 before this code is called. (Limitation of the override mechanism of ARMCC) 
+		*/
+		//TODO: Look for a more elegant fix.
+    //CMU_ClockSelectSet(cmuClock_LFB, LFXO);
 #endif
 #ifdef CMU_LFECLKSEL_REG
     CMU_ClockSelectSet(cmuClock_LFE, LFXO);

libraries/mbed/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/serial_api.c

@@ -407,6 +407,7 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
     if(LEUART_REF_VALID(obj->serial.periph.leuart)) {
         // Set up LEUART clock tree to use high-speed clock)
         CMU_ClockSelectSet(cmuClock_LFB, cmuSelect_CORELEDIV2);
+			  CMU_ClockEnable(cmuClock_LFB, true);
         CMU_ClockSelectSet(serial_get_clock(obj), cmuSelect_CORELEDIV2);
     }