iio: dac: ad7303: Add OF match table

The Documentation/devicetree/bindings/iio/dac/ad7303.txt DT binding doc
lists "adi,ad7303" as a compatible string but the corresponding driver
does not have an OF match table. Add the table to the driver so the SPI
core can do an OF style match.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>