usb: host: xhci-tegra: Add support for XUSB context save/restore

The XUSB controller contains registers that need to be saved on suspend
and restored on resume in addition to the XHCI specific registers. Add
support for saving and restoring the XUSB specific context.

Based on work by JC Kuo <jckuo@nvidia.com>.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191206140653.2085561-9-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>