dt-bindings: iio: dac: adi,ad5064: Document bindings for many different DACs

Note this is documenting bindings that have effectively existing ever
since this driver was merged. The naming conventions for the ADI
parts are inconsistent on the data sheets which has lead to a mixture
of -X and -reference voltage part naming.  We could attempt to clean this
up, but as we are stuck supporting the existing binding it is probably
not worthwhile.

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/20210627163244.1090296-3-jic23@kernel.org