drivers/dac: stm32: Enabling DAC Loopback tests for stm32f3_disco

This commit enables dac_loopback to build and run on stm32f3_disco
platform.
This has been tested with twister and also as a standalone
dac_loopback test and is working as expected.

Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>