of: Add NVIDIA Tegra XUSB pad controller binding

This patch adds the device tree binding documentation for the XUSB pad
controller found on NVIDIA Tegra SoCs. It exposes both pinmuxing and PHY
capabilities.

Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>