MAINTAINERS: add a keyword entry for the GPIO subsystem

Every now and then - despite being clearly documented as deprecated -
the legacy GPIO API is being used in some new drivers in the kernel. Add
a keyword pattern matching the unwanted functions so that I get Cc'ed
anytime they're being used and get the chance to object.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20241017071835.19069-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>