dt-bindings: usb: Add NVIDIA Tegra XUSB device mode controller binding

Add device-tree binding documentation for the XUSB device mode controller
present on Tegra210 and Tegra186 SoC. This controller supports the USB 3.0
specification.

Signed-off-by: Nagarjuna Kristam <nkristam@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>