dt-bindings: iio: dac: ad5761: Add missing binding doc.

Straight forward binding with vref optional for those part with internal
regulators, but required for those without.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ricardo Ribalda <ribalda@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-12-jic23@kernel.org