USB: Add documentation for USB suspend statistics.

This documents two newly created files:
/sys/bus/usb/device/.../power/connected_duration
/sys/bus/usb/device/.../power/active_duration

Documentation was placed in Documentation/ABI/testing, since that's where the
documentation is for the other USB sysfs power files.

Signed-off-by: Sarah Sharp <sarah.a.sharp@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>