dt-bindings: iio: dac: adi,ad8801: Add missing binding document.

This device was missing a binding document. Simple binding with the
ad8803 requiring two both low and high references whilst the 8801
only has a high reference.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-15-jic23@kernel.org