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

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

@@ -38,6 +38,14 @@ properties:
       The first value specifies the positive input pin, the second
       specifies the negative input pin.
 
+  single-channel:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      When devices combine single-ended and differential channels, allow the
+      channel for a single element to be specified, independent of reg (as for
+      differential channels). If this and diff-channels are not present reg
+      shall be used instead.
+
   settling-time-us:
     description:
       Time between enabling the channel and first stable readings.
@@ -50,4 +58,15 @@ properties:
       device design and can interact with other characteristics such as
       settling time.
 
+anyOf:
+  - oneOf:
+      - required:
+          - reg
+          - diff-channels
+      - required:
+          - reg
+          - single-channel
+  - required:
+      - reg
+
 additionalProperties: true