MAINTAINERS: GPIO: Add Documentation/driver-api/gpio/

Steer patches to Documentation/driver-api/gpio/ into the right
direction.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -6029,6 +6029,7 @@ L:	linux-gpio@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/gpio/
+F:	Documentation/driver-api/gpio/
 F:	Documentation/gpio/
 F:	Documentation/ABI/testing/gpio-cdev
 F:	Documentation/ABI/obsolete/sysfs-gpio