dt-bindings: iio: adc: document diff-channels corner case for some ADCs

Some ADCs have channels with negative and positive inputs, which can be
used to measure differential voltage levels. These inputs/pins are
dedicated (to the given channel) and cannot be muxed as with other ADCs.

For those types of setups, the 'diff-channels' property can be specified to
be used with the channel number (or reg property) for both negative and
positive inputs/pins.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240919130444.2100447-7-aardelean@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -37,6 +37,10 @@ properties:
       to both the positive and negative inputs of a differential ADC.
       The first value specifies the positive input pin, the second
       specifies the negative input pin.
+      There are also some ADCs, where the differential channel has dedicated
+      positive and negative inputs which can be used to measure differential
+      voltage levels. For those setups, this property can be configured with
+      the 'reg' property for both inputs (i.e. diff-channels = <reg reg>).
 
   single-channel:
     $ref: /schemas/types.yaml#/definitions/uint32