iio: as3935: Add OF match table

The Documentation/devicetree/bindings/iio/proximity/as3935.txt DT binding
doc lists "ams,as3935" 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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>