dt-bindings: gpio: Document GPIOs via Moxtet bus

This patch adds documentation of the device tree bindings for GPIOs
on the devices connected via Moxtet bus.

Link: https://lore.kernel.org/r/20190812161118.21476-6-marek.behun@nic.czSigned-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/gpio/gpio-moxtet.txt

+Turris Mox Moxtet GPIO expander via Moxtet bus
+
+Required properties:
+ - compatible		: Should be "cznic,moxtet-gpio".
+ - gpio-controller	: Marks the device node as a GPIO controller.
+ - #gpio-cells		: Should be two. For consumer use see gpio.txt.
+
+Other properties are required for a Moxtet bus device, please refer to
+Documentation/devicetree/bindings/bus/moxtet.txt.
+
+Example:
+
+	moxtet_sfp: gpio@0 {
+		compatible = "cznic,moxtet-gpio";
+		gpio-controller;
+		#gpio-cells = <2>;
+		reg = <0>;
+	}

MAINTAINERS

@@ -1633,6 +1633,7 @@ S:	Maintained
 F:	Documentation/ABI/testing/debugfs-moxtet
 F:	Documentation/ABI/testing/sysfs-bus-moxtet-devices
 F:	Documentation/devicetree/bindings/bus/moxtet.txt
+F:	Documentation/devicetree/bindings/gpio/gpio-moxtet.txt
 F:	include/linux/moxtet.h
 F:	drivers/bus/moxtet.c
 F:	drivers/gpio/gpio-moxtet.c