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