PM: add /sys/power documentation to Documentation/ABI

The file sysfs-power that documents the interface in the /sys/power/ directory
is added to Documentation/ABI/testing.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>