Add support for the on-chip XUSB controller present on Tegra SoCs. This
controller, when loaded with external firmware, exposes an interface
compliant with xHCI. This driver loads the firmware, starts the
controller, and is able to service host-specific messages sent by the
controller's firmware.

The controller also supports USB device mode as well as powergating
of the SuperSpeed and host-controller logic when not in use, but
support for these is not yet implemented.

Based on work by:
  Ajay Gupta <ajayg@nvidia.com>
  Bharath Yadav <byadav@nvidia.com>
  Andrew Bresticker <abrestic@chromium.org>

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>