dm: tegra: Bring in GPIO device tree binding

At present the tegra GPIO driver does not fully support the existing device
tree binding, but add the binding file to cover the existing partial support.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/gpio/nvidia,tegra20-gpio.txt

@@ -0,0 +1,40 @@
+NVIDIA Tegra GPIO controller
+
+Required properties:
+- compatible : "nvidia,tegra<chip>-gpio"
+- reg : Physical base address and length of the controller's registers.
+- interrupts : The interrupt outputs from the controller. For Tegra20,
+  there should be 7 interrupts specified, and for Tegra30, there should
+  be 8 interrupts specified.
+- #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.
+- #interrupt-cells : Should be 2.
+  The first cell is the GPIO number.
+  The second cell is used to specify flags:
+    bits[3:0] 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.
+      Valid combinations are 1, 2, 3, 4, 8.
+- interrupt-controller : Marks the device node as an interrupt 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;
+	#interrupt-cells = <2>;
+	interrupt-controller;
+};