MAINTAINERS: add entry OMAP GPIO driver

Since I've been maintaining this, making it official at the request of the
GPIO maintainers.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kevin Hilman <khilman@ti.com>
[corrected file path]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>