dt-bindings: iio: adc: Add common-mode-channel property

dt-bindings: iio: adc: Add common-mode-channel property

There are ADCs that are differential but support to measure single-ended
signals on the same channels by connecting a constant voltage to the
negative input pin.

This property allows to properly define a single-ended channel that
requires two inputs to be specified. Software can use the presence of
this property to mark the channel as not differential.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240607-ad4111-v7-1-97e3855900a0@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>