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

Documentation/devicetree/bindings/iio/adc/adc.yaml

@@ -46,6 +46,17 @@ properties:
       differential channels). If this and diff-channels are not present reg
       shall be used instead.
 
+  common-mode-channel:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Some ADCs have differential input pins that can be used to measure
+      single-ended or pseudo-differential inputs. This property can be used
+      in addition to single-channel to signal software that this channel is
+      not differential but still specify two inputs.
+
+      The input pair is specified by setting single-channel to the positive
+      input pin and common-mode-channel to the negative pin.
+
   settling-time-us:
     description:
       Time between enabling the channel and first stable readings.