dt-bindings:iio:adc:adi,ad7476: Add missing binding document

This binding covers class of simple SPI ADCs which only provide
data output - they don't have MOSI pin.

The only real variation between them is over how many supplies they
use and which one is used for the reference.

Michael listed as maintainer for this one as it is his driver and
falls under the catch all MAINTAINERS entry for ADI devices.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210425163154.73209-3-jic23@kernel.org