dts: bindings: adc-controller: Add zephyr,differential property

Add a property that allows explicit selection of the differential
input mode for ADC channels in DTS. This is useful for controllers
that do not have configurable inputs, so the zephyr,negative-input
property that implicitly selects the differential mode is not
specified for them.

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