dt-bindings: iio: dac: ad5624r: Add missing binding document

Simple binding, with optional vref.
Note that the Linux driver does support vcc-supply for historical
reasons, but lets not let that get into any bindings that are checked
going forwards.  Hence I have deliberately not documented it.

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/20210627163244.1090296-10-jic23@kernel.org