of: Add NVIDIA Tegra flow controller bindings

Add device tree bindings for the flow controller found on NVIDIA Tegra
SoCs.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NStephen Warren <swarren@nvidia.com>

Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-flowctrl.txt

+NVIDIA Tegra Flow Controller
+
+Required properties:
+- compatible: Should be "nvidia,tegra<chip>-flowctrl"
+- reg: Should contain one register range (address and length)
+
+Example:
+
+	flow-controller@60007000 {
+		compatible = "nvidia,tegra20-flowctrl";
+		reg = <0x60007000 0x1000>;
+	};