soc/tegra: pmc: Enable XUSB partitions on boot

The Tegra XHCI driver does not currently manage the Tegra XUSB power
partitions and so it these partitions have not been enabled by the
bootloader then the system will crash when probing the XHCI device.

While proper support for managing the power partitions is being
developed to the XHCI driver for Tegra, for now power on all the XUSB
partitions for USB host and super-speed on boot if the XHCI driver is
enabled.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>