usb: xhci: tegra: Add support for managing powergates

The Tegra XHCI controller requires that the XUSBA (for superspeed) and
XUSBC (for host) power-domains are enabled. Commit 8df12745


("soc/tegra: pmc: Enable XUSB partitions on boot") was added to force
on these power-domains if the XHCI driver is enabled while proper
power-domain support is added, to ensure the device did not hang on
boot. However, rather than forcing on these power-domains in the PMC
driver we can use the legacy Tegra powergate APIs to turn on these
power-domains during the probe of the Tegra XHCI driver.

In the near future we plan to move the Tegra XHCI driver to use the
generic PM domain framework for power-domains and so to prepare for
this only use the legacy Tegra powergate API if there is not PM
domain associated with device (ie. dev.pm_domain is NULL). Please
note that in the future the superspeed and host resets will be handled
by the generic PM domain provider and so these are only these are only
needed in the case where there is no generic PM domain.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>