mfd: cros_ec: spi: Add OF match table

The Documentation/devicetree/bindings/mfd/cros-ec.txt DT binding doc lists
"google,cros-ec-spi" 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: Lee Jones <lee.jones@linaro.org>