usb: host: xhci-tegra: Implement basic ELPG support

This implements basic engine-level powergate support which allows the
XUSB controller to be put into a low power mode on system sleep and get
it out of that low power mode again on resume.

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

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191206140653.2085561-11-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>