stm32f7: enable existing IWDG support

lib/stm32/f7/Makefile

@@ -47,6 +47,7 @@ OBJS		+= dma_common_f24.o
 OBJS		+= flash_common_all.o flash_common_f.o flash_common_f24.o flash.o
 OBJS		+= gpio.o gpio_common_all.o gpio_common_f0234.o
 OBJS		+= i2c_common_v2.o
+OBJS		+= iwdg_common_all.o
 OBJS		+= pwr.o rcc.o
 OBJS		+= rcc_common_all.o
 OBJS		+= rng_common_v1.o