dt-bindings: iio: adc: Add single-channel property

Devices that have both single-ended channels and differential channels
cause a bit of confusion when the channels are configured in the
devicetree.

Clarify difference between these two types of channels for such devices
by adding single-channel property alongside diff-channels. They should
be mutually exclusive.

Devices that have only single-ended channels can still use reg property
to reference a channel like before.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240514120222.56488-5-alisa.roman@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>