fdt: Add tegra-usb bindings file from linux

This file is taken from the Linux mailing list.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>

doc/device-tree-bindings/usb/tegra-usb.txt

+Tegra SOC USB controllers
+
+The device node for a USB controller that is part of a Tegra
+SOC is as described in the document "Open Firmware Recommended
+Practice : Universal Serial Bus" with the following modifications
+and additions :
+
+Required properties :
+ - compatible : Should be "nvidia,tegra20-ehci" for USB controllers
+   used in host mode.
+ - phy_type : Should be one of "ulpi" or "utmi".
+ - nvidia,vbus-gpio : If present, specifies a gpio that needs to be
+   activated for the bus to be powered.