dt-bindings: iio: dac: adi,ad5360: Add missing binding document

Bindings for the family of many channel DACs.  Fairly straight forward
with just a differing number of voltage references (an extra one
for the 40 channel ad4371)

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/20210627163244.1090296-4-jic23@kernel.org