MAINTAINERS: update GPIO OMAP driver entry

MAINTAINERS: update GPIO OMAP driver entry

I've been maintaining this driver by fixing all issues
found while migrating OMAP2+ towards Device Tree based
booting and keeping it up-to-date by using the latest
infraestructure that is provided by the GPIO subsystem.

It would be nice if people know that I care about this
driver and put me in copy when sending patches for it.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>