dt-bindings:iio:cdc:adi,ad7150 binding doc

Binding covering the ad7150, ad7151 and ad7156 capacitance to digital
convertors.  The only difference between these is how many channels they
have (1 or 2)

Whilst it is clearly necessary to provide power to the part, we don't
need to know the voltage or anything so if it is always on, there should
be no need to have it specified in the binding.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210314181511.531414-24-jic23@kernel.org