drivers: adc: add dt-spec helper functions

This patch adds the following dt-spec helpers:

- adc_channel_setup_dt()
- adc_raw_to_millivolts_dt()
- adc_sequence_init_dt()

The objective of these functions is to reduce application boilerplate
when Devicetree is used to configure the ADC.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>