dt-bindings:iio:adc:adi,ad7292: Use new adc.yaml binding for channels.

Also add additionalProperties: false for the child nodes.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20201031182423.742798-4-jic23@kernel.org

Documentation/devicetree/bindings/iio/adc/adi,ad7292.yaml

@@ -45,10 +45,10 @@ required:
 
 patternProperties:
   "^channel@[0-7]$":
+    $ref: "adc.yaml"
     type: object
     description: |
       Represents the external channels which are connected to the ADC.
-      See Documentation/devicetree/bindings/iio/adc/adc.txt.
 
     properties:
       reg:
@@ -58,13 +58,13 @@ patternProperties:
           - minimum: 0
             maximum: 7
 
-      diff-channels:
-        description: see Documentation/devicetree/bindings/iio/adc/adc.txt
-        maxItems: 1
+      diff-channels: true
 
     required:
       - reg
 
+    additionalProperties: true
+
 additionalProperties: false
 
 examples: