dt-bindings: iio: dac: adi,ad5791: Add missing bindings document

Documenting existing bindings for this device.

The Linux driver in theory 'works' without the two supplies that I have
listed as required, but without a valid scale.  As such I've
documented it as required.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-14-jic23@kernel.org