MAINTAINERS: gpio: add DT bindings directory

Helps get_maintainer.pl to find the right people.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -4884,6 +4884,7 @@ M:	Alexandre Courbot <gnurou@gmail.com>
 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/gpio/
 F:	Documentation/ABI/testing/gpio-cdev
 F:	Documentation/ABI/obsolete/sysfs-gpio