dt-bindings:iio:adc:adi,ad7124: Use the new adc.yaml channel binding

This both ensures this binding is compliant with the generic properties
and reduces the amount we need to specify in this separate binding.

Whilst here mark the child node as additionalProperties: false

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Link: https://lore.kernel.org/r/20201031182423.742798-3-jic23@kernel.org