MAINTAINERS: add Documentation/userspace-api/gpio/ to GPIO UAPI section

Add Documentation/userspace-api/gpio/ to the GPIO UAPI section.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

MAINTAINERS

@@ -9148,6 +9148,7 @@ S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/brgl/linux.git
 F:	Documentation/ABI/obsolete/sysfs-gpio
 F:	Documentation/ABI/testing/gpio-cdev
+F:	Documentation/userspace-api/gpio/
 F:	drivers/gpio/gpiolib-cdev.c
 F:	include/uapi/linux/gpio.h
 F:	tools/gpio/