usbcore: add sysfs support to xHCI usb2 hardware LPM

This patch adds sysfs support to xHCI usb2 hardware LPM, so developer can
enable and disable usb2 hardware LPM manually for test purpose.

Signed-off-by: Andiry Xu <andiry.xu@amd.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>