ARM: tegra: add DT nodes for Tegra USB PHY

Add DT nodes for Tegra USB PHY along with related documentation.
Also added a phandle property to controller DT node, for referring
to connected PHY instance.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>