dt-bindings: usb: xhci-tegra: Add power-domain details

Add details for power-domains to the Tegra xHCI bindings so that
generic power-domains can be used for inconjunction with the xHCI
driver.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>