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>

Author: Bartosz Golaszewski

MAINTAINERS

@@ -9745,6 +9745,7 @@ F:	include/dt-bindings/gpio/
 F:	include/linux/gpio.h
 F:	include/linux/gpio/
 F:	include/linux/of_gpio.h
+K:	(devm_)?gpio_(request|free|direction|get|set)
 
 GPIO UAPI
 M:	Bartosz Golaszewski <brgl@bgdev.pl>