MAINTAINERS: add include/dt-bindings/gpio to GPIO SUBSYSTEM

Maintainers of the directory Documentation/devicetree/bindings/gpio
are also the maintainers of the corresponding directory
include/dt-bindings/gpio.

Add the file entry for include/dt-bindings/gpio to the appropriate
section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>