samples/drivers/adc: Add support for nRF52840 DK and nRF51 DK boards

The overlay for the nRF52840 DK board uses ADC controller child nodes
for specifying channel configurations and it shows how to configure
a channel in differential input mode, so it can serve as an example
of how to use these new possibilities. The overlay for the nRF51 DK
board shows how to use a specific ADC resolution (the default 12 bit
setting is not supported by nRF51 SoCs).

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