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>

MAINTAINERS

@@ -8479,6 +8479,7 @@ F:	Documentation/devicetree/bindings/gpio/
 F:	Documentation/driver-api/gpio/
 F:	drivers/gpio/
 F:	include/asm-generic/gpio.h
+F:	include/dt-bindings/gpio/
 F:	include/linux/gpio.h
 F:	include/linux/gpio/
 F:	include/linux/of_gpio.h