T60: undock on external power loss

T60: undock on external power loss

If power is unplugged/lost, we should undock the docking station.
The power loss can also be caused by the fact that the user removed
the thinkpad from the docking station without pressing the Undock button/hotkey
first. Without undocking it on this event, the thinkpad LPC switch will still
connect the Docking connector, which causes crashes when docking it again.

Change-Id: I9ed783e491827bde20264868eab2b3a79c232922
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/62
Tested-by: build bot (Jenkins)