stm32: extract g0 exti stuff to exti_common_v2.

stm32l5 basically uses the same stuff.

lib/stm32/g0/Makefile

@@ -37,7 +37,7 @@ OBJS += adc.o adc_common_v2.o
 OBJS += crc_common_all.o
 OBJS += dma_common_l1f013.o
 OBJS += dmamux.o
-OBJS += exti.o exti_common_all.o
+OBJS += exti_common_all.o exti_common_v2.o
 OBJS += flash.o flash_common_all.o
 OBJS += gpio_common_all.o gpio_common_f0234.o
 OBJS += i2c_common_v2.o