boards: opta: ADC support

A valid device tree configuration is provided for the ADCs of the 8 input
channels and the sample adc_dt works out of the box. Obviously this is
not the only possible configuration but it provides a good template for
further customization without the need to lookup the ADC GPIOs and
connections in the schematics.

Signed-off-by: Federico Di Gregorio <fog@dndg.it>