TARGET_EFM32GG - fix ISR stack size (IAR)

Author: mprse

targets/TARGET_Silicon_Labs/TARGET_EFM32/TARGET_EFM32GG/device/TARGET_1024K/TOOLCHAIN_IAR/efm32gg990f1024.icf

@@ -15,8 +15,8 @@ define symbol __NVIC_end__                   = 0x200000DB;
 define symbol __ICFEDIT_region_RAM_start__   = 0x200000DC;
 define symbol __ICFEDIT_region_RAM_end__     = 0x2001FFFF;
 /*-Sizes-*/
-/*Heap 1/4 of ram and stack 1/8*/
-define symbol __ICFEDIT_size_cstack__   = 0x4000;
+/*Heap 1/4 of ram and stack 1024 B*/
+define symbol __ICFEDIT_size_cstack__   = 0x400;
 define symbol __ICFEDIT_size_heap__     = 0x8000;
 /**** End of ICF editor section. ###ICF###*/