usbcore: add sysfs support to xHCI usb3 hardware LPM

Add a sysfs node to make it easier to verify if LPM is supported and being
enabled for USB 3.0 devices.

Signed-off-by: Kevin Strasser <kevin.strasser@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc Documentation/ABI/testing/sysfs-bus-usb
Simple merge

diff --cc Documentation/usb/power-management.txt
Simple merge

diff --cc drivers/usb/core/hub.c
Simple merge

diff --cc drivers/usb/core/sysfs.c
Simple merge