dt: tegra gpio: Flesh out binding documentation

Document the required reg and interrupts properties.
Add a complete example.
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Acked-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NOlof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/gpio/gpio_nvidia.txt

@@ -2,7 +2,25 @@ NVIDIA Tegra 2 GPIO controller
 
 Required properties:
 - compatible : "nvidia,tegra20-gpio"
+- reg : Physical base address and length of the controller's registers.
+- interrupts : The interrupt outputs from the controller.
 - #gpio-cells : Should be two. The first cell is the pin number and the
   second cell is used to specify optional parameters:
   - bit 0 specifies polarity (0 for normal, 1 for inverted)
 - gpio-controller : Marks the device node as a GPIO controller.
+
+Example:
+
+gpio: gpio@6000d000 {
+	compatible = "nvidia,tegra20-gpio";
+	reg = < 0x6000d000 0x1000 >;
+	interrupts = < 0 32 0x04
+		       0 33 0x04
+		       0 34 0x04
+		       0 35 0x04
+		       0 55 0x04
+		       0 87 0x04
+		       0 89 0x04 >;
+	#gpio-cells = <2>;
+	gpio-controller;
+};