dt-bindings: phy: tegra-xusb: Add nvidia,pmc prop

This commit describes the "nvidia,pmc" property for Tegra210 tegra-xusb
PHY driver. It is a phandle and specifier referring to the Tegra210
pmc@7000e400 node.

Signed-off-by: JC Kuo <jckuo@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>