dt-bindings: iio: dac: ad5686 and ad5696: Add missing binding document.

The use of vcc-supply for the reference voltage is unusual and should
probably be deprecated as there is an explicit VREF pin on at least
some of these parts.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-11-jic23@kernel.org