dt-bindings: iio: adc: adi,ad7292: Fix additionalProperties on channel nodes

"additionalProperties: true" is only for incomplete schemas such as bus
child nodes in a bus's schema. That doesn't apply to the "channel" nodes
in the adi,ad7292 binding, so fix additionalProperties to be false.

Signed-off-by: Rob Herring <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Acked-by: Marcelo Schmitt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

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

···
     required:
       - reg
 
-    additionalProperties: true
+    additionalProperties: false
 
 allOf:
   - $ref: /schemas/spi/spi-peripheral-props.yaml#