MAINTAINERS: update the GPIO git tree entry

My git tree has become the de facto main GPIO tree. Update the
MAINTAINERS file to reflect that.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reported-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>