usb: dwc2: add controller hibernation support

When suspending usb bus, phy driver may disable controller power.
In this case, registers need to be saved on suspend and restored
on resume.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>