Documentation: gpio: Add APM X-Gene SoC GPIO controller DTS binding

Documentation for APM X-Gene SoC GPIO controller DTS binding.
Signed-off-by: NFeng Kan <fkan@apm.com>
Reviewed-by: NAlexandre Courbot <acourbot@nvidia.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

+APM X-Gene SoC GPIO controller bindings
+
+This is a gpio controller that is part of the flash controller.
+This gpio controller controls a total of 48 gpios.
+
+Required properties:
+- compatible: "apm,xgene-gpio" for X-Gene GPIO controller
+- reg: Physical base address and size of the controller's registers
+- #gpio-cells: Should be two.
+	- first cell is the pin number
+	- second cell is used to specify the gpio polarity:
+		0 = active high
+		1 = active low
+- gpio-controller: Marks the device node as a GPIO controller.
+
+Example:
+	gpio0: gpio0@1701c000 {
+		compatible = "apm,xgene-gpio";
+		reg = <0x0 0x1701c000 0x0 0x40>;
+		gpio-controller;
+		#gpio-cells = <2>;
+	};