usb: xhci: tegra: Add genpd support

The generic power-domain framework has been updated to allow devices
that require more than one power-domain to create a new device for
each power-domain required and then link these new power-domain
devices to the consumer device.

Update the Tegra xHCI driver to use the new APIs provided by the
generic power-domain framework so we can use the generic power-domain
framework for managing the xHCI controllers power-domains. Please
note that to maintain backward compatibility with older device-tree
blobs these new generic power-domain APIs are only used if the
'power-domains' property is present and otherwise we fall back to
using the legacy Tegra APIs for managing the power-domains.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>