dt-bindings: iio: adc: ad7292: fix constraint over channel quantity

Change items property of AD7292 channels to correctly constrain their
quantity.
Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -53,7 +53,8 @@ patternProperties:
         description: |
           The channel number. It can have up to 8 channels numbered from 0 to 7.
         items:
-          maximum: 7
+          - minimum: 0
+            maximum: 7
 
       diff-channels:
         description: see Documentation/devicetree/bindings/iio/adc/adc.txt