samples: drivers: adc: configure ADC channel/s in Devicetree

It is now possible to configure ADC channels in Devicetree. This makes
code more portable, since there is no need to hardcode hardware specific
bits in the C sources.

This patch adds channel/s configuration for all support sample overlays.
Hardcoded values have been used:

- Resolution: 12-bit
- Gain: 1
- Reference: internal
- Acquisition time: default

Some nRF channels have been adjusted to include resolution as well.

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