dts: Include ADC DT binding headers from nRF SoC dtsi files

Add inclusions of header files with devicetree related ADC definitions
to the nRF SoC dtsi files so that those definitions can be used also
for nRF SoC based boards.
Provide definitions of nRF ADC and SAADC analog inputs suitable
for use in devicetree.

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