MAINTAINERS: Update OMAP GPIO driver entry

I'm not as involved in OMAP as I used to be and Grygorii is the most
active developer working on this driver and has also been reviewing
patches so let's add him as a driver maintainer instead of me.

While being there, add the driver's DT binding doc to the file list.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>