Fixed wrong FreeRTOS port for EFM32GG12

Author: silabs-hfegran

hw/bsp/sltb009a/board.mk

@@ -35,7 +35,7 @@ INC += \
   $(TOP)/hw/bsp/$(BOARD)
 
 # For freeRTOS port source
-FREERTOS_PORT = ARM_CM3
+FREERTOS_PORT = ARM_CM4F
 
 # For flash-jlink target
 JLINK_DEVICE = EFM32GG12B810F1024