dt-bindings: iio: dac: ad5449: Add missing binding document.

Documenting existing binding, so little flexibility available.
2 channel devices that require separate reference voltages.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-7-jic23@kernel.org