Documentation: dts: add bindings for Vybrid GPIO/PORT module

The Vybrid SoC device tree (vf610.dtsi) used this bindings since
its initial commit in May 2013. However, a proper gpiolib driver
was missing so far. With the addition of the gpiolib driver,
the bindings proved to be useful and complete, hence a good time
to add the documentation.
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-vf610.txt

+* Freescale VF610 PORT/GPIO module
+
+The Freescale PORT/GPIO modules are two adjacent modules providing GPIO
+functionality. Each pair serves 32 GPIOs. The VF610 has 5 instances of
+each, and each PORT module has its own interrupt.
+
+Required properties for GPIO node:
+- compatible : Should be "fsl,<soc>-gpio", currently "fsl,vf610-gpio"
+- reg : The first reg tuple represents the PORT module, the second tuple
+  the GPIO module.
+- interrupts : Should be the port interrupt shared by all 32 pins.
+- gpio-controller : Marks the device node as a gpio controller.
+- #gpio-cells : Should be two. The first cell is the pin number and
+  the second cell is used to specify the gpio polarity:
+      0 = active high
+      1 = active low
+- interrupt-controller: Marks the device node as an interrupt controller.
+- #interrupt-cells : Should be 2.  The first cell is the GPIO number.
+  The second cell bits[3:0] is used to specify trigger type and level flags:
+      1 = low-to-high edge triggered.
+      2 = high-to-low edge triggered.
+      4 = active high level-sensitive.
+      8 = active low level-sensitive.
+
+Note: Each GPIO port should have an alias correctly numbered in "aliases"
+node.
+
+Examples:
+
+aliases {
+	gpio0 = &gpio1;
+	gpio1 = &gpio2;
+};
+
+gpio1: gpio@40049000 {
+	compatible = "fsl,vf610-gpio";
+	reg = <0x40049000 0x1000 0x400ff000 0x40>;
+	interrupts = <0 107 IRQ_TYPE_LEVEL_HIGH>;
+	gpio-controller;
+	#gpio-cells = <2>;
+	interrupt-controller;
+	#interrupt-cells = <2>;
+	gpio-ranges = <&iomuxc 0 0 32>;
+};
+
+gpio2: gpio@4004a000 {
+	compatible = "fsl,vf610-gpio";
+	reg = <0x4004a000 0x1000 0x400ff040 0x40>;
+	interrupts = <0 108 IRQ_TYPE_LEVEL_HIGH>;
+	gpio-controller;
+	#gpio-cells = <2>;
+	interrupt-controller;
+	#interrupt-cells = <2>;
+	gpio-ranges = <&iomuxc 0 32 32>;
+};