stm32l4: adc: Initial support for the adc-v2 periph

Now that there's an adc-v2 peripheral layer, we can just use it straight away
for L4.

lib/stm32/l4/Makefile

@@ -42,6 +42,7 @@ OBJS		=
 # common/shared objs
 OBJS            += rcc_common_all.o
 OBJS            += gpio_common_all.o gpio_common_f0234.o
+OBJS            += adc_common_v2.o
 
 VPATH += ../../usb:../:../../cm3:../common
 VPATH += ../../ethernet