samples: drivers: adc: example to use AD4114 on stm32f3_disco

This examples shows how to use the AD4114 ADC on a
stm32f3_disco board.
As the AD4114 is a 24bits ADC resolution we need to be able
to pass 32bits buffer to get the samples.
CONFIG_SEQUENCE_RESOLUTION is set to 12 instead of 24, and the
gain zephyr,vref-mv is adapted to avoid overflows using the
exisiting adc_raw_to_millivolts function.

Signed-off-by: Pierrick Curt <pierrickcurt@gmail.com>