samples/drivers/adc: Rework to use channel configurations from DT

Allow specifying configuration of channels through child nodes of ADC
controllers. This way analog inputs can be specified for channels in
SoCs that require this and it is also possible, for example, to use
different gain or reference selections for particular channels.

This commit also removes a few former limitations of the sample: now
it is possible to use more than 2 channels and each channel can use
a different ADC controller. Also the ADC resolution to be used can be
specified through devicetree.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>