DT: Add documentation for gpio-xilinx

Describe gpio-xilinx binding.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

+Xilinx plb/axi GPIO controller
+
+Dual channel GPIO controller with configurable number of pins
+(from 1 to 32 per channel). Every pin can be configured as
+input/output/tristate. Both channels share the same global IRQ but
+local interrupts can be enabled on channel basis.
+
+Required properties:
+- compatible : Should be "xlnx,xps-gpio-1.00.a"
+- reg : Address and length of the register set for the device
+- #gpio-cells : Should be two. The first cell is the pin number and the
+  second cell is used to specify optional parameters (currently unused).
+- gpio-controller : Marks the device node as a GPIO controller.
+
+Optional properties:
+- interrupts : Interrupt mapping for GPIO IRQ.
+- interrupt-parent : Phandle for the interrupt controller that
+  services interrupts for this device.
+- xlnx,all-inputs : if n-th bit is setup, GPIO-n is input
+- xlnx,dout-default : if n-th bit is 1, GPIO-n default value is 1
+- xlnx,gpio-width : gpio width
+- xlnx,tri-default : if n-th bit is 1, GPIO-n is in tristate mode
+- xlnx,is-dual : if 1, controller also uses the second channel
+- xlnx,all-inputs-2 : as above but for the second channel
+- xlnx,dout-default-2 : as above but the second channel
+- xlnx,gpio2-width : as above but for the second channel
+- xlnx,tri-default-2 : as above but for the second channel
+
+
+Example:
+gpio: gpio@40000000 {
+	#gpio-cells = <2>;
+	compatible = "xlnx,xps-gpio-1.00.a";
+	gpio-controller ;
+	interrupt-parent = <&microblaze_0_intc>;
+	interrupts = < 6 2 >;
+	reg = < 0x40000000 0x10000 >;
+	xlnx,all-inputs = <0x0>;
+	xlnx,all-inputs-2 = <0x0>;
+	xlnx,dout-default = <0x0>;
+	xlnx,dout-default-2 = <0x0>;
+	xlnx,gpio-width = <0x2>;
+	xlnx,gpio2-width = <0x2>;
+	xlnx,interrupt-present = <0x1>;
+	xlnx,is-dual = <0x1>;
+	xlnx,tri-default = <0xffffffff>;
+	xlnx,tri-default-2 = <0xffffffff>;
+} ;