stm32l4: add common DAC support

Replace the DAC1_BASE style, only used on l4 with the standard DAC_BASE
used on all other targets.

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

lib/stm32/l4/Makefile

@@ -54,6 +54,7 @@ OBJS            += dma_common_l1f013.o
 OBJS            += iwdg_common_all.o
 OBJS            += rtc_common_l1f024.o
 OBJS            += spi_common_all.o spi_common_v2.o
+OBJS            += dac_common_all.o
 
 OBJS            += usb.o usb_control.o usb_standard.o usb_msc.o
 OBJS            += st_usbfs_core.o st_usbfs_v2.o