[EFM32] Collapse NVIC relocation handling

Gecko SDK 5.0.0 provides a convenient define for the amount of vectors wired on the chip, so we can use that to collapse the cmsis_nvic.h header

Author: stevew817

targets/TARGET_Silicon_Labs/TARGET_EFM32/TARGET_EFM32HG/device/cmsis_nvic.h

@@ -9,7 +9,7 @@
 
 #include "cmsis.h"
 
-#define NVIC_NUM_VECTORS      (16 + 39)   // CORE + MCU Peripherals
+#define NVIC_NUM_VECTORS      (16 + EXT_IRQ_COUNT)   // CORE + MCU Peripherals
 #define NVIC_USER_IRQ_OFFSET  16
 
 #ifdef __cplusplus