iio: adc128s052: Add OF match table

The Documentation/devicetree/bindings/iio/adc/ti-adc128s052.txt DT binding
doc lists "ti,adc128s052" or "ti,adc122s021" as compatible strings 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>