stm32g4: add IWDG support

Reviewed-by: Karl Palsson <karlp@tweak.au>

lib/stm32/g4/Makefile

@@ -47,6 +47,7 @@ OBJS += fdcan.o fdcan_common.o
 OBJS += flash.o flash_common_all.o flash_common_f.o flash_common_idcache.o
 OBJS += gpio_common_all.o gpio_common_f0234.o
 OBJS += i2c_common_v2.o
+OBJS += iwdg_common_all.o
 OBJS += opamp_common_all.o opamp_common_v2.o
 OBJS += pwr.o
 OBJS += rcc.o rcc_common_all.o