gpio: ixp4xx: Add DT bindings

This adds DT bindings for the IXP4xx GPIO controller.

Cc: devicetree@vger.kernel.org
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -1690,6 +1690,7 @@ M:	Imre Kaloz <kaloz@openwrt.org>
 M:	Krzysztof Halasa <khalasa@piap.pl>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
+F:	Documentation/devicetree/bindings/gpio/intel,ixp4xx-gpio.txt
 F:	Documentation/devicetree/bindings/interrupt-controller/intel,ixp4xx-interrupt.yaml
 F:	Documentation/devicetree/bindings/timer/intel,ixp4xx-timer.yaml
 F:	arch/arm/mach-ixp4xx/