drivers/platform/x86/thinkpad_acpi.c: Handle HKEY event 0x6040

Handle HKEY event generated on AC power change. The current message
asks users to submit data related to this event which leads to
a lot of confusion and noise on the mailing list.

The following is a list affected models and 'Message-Id'
from ibm-acpi-devel@lists.sourceforge.net of people who saw this
event when either plugging or unplugging the AC adapter or
docking or undocking their laptop.

X120e - CAAAujb5v9dHdbdxDVvhNJoG4UrZC1TgKqeB_zGpAy7q8kZHMEQ@mail.gmail.com
X121e - 20120817143459.GB3462@x1.osrc.amd.com
X220  - Confirmed by Richard Hartmann
X220i - 4F406274.7070807@gmail.com
X220t - 4F489F5B.9040705@cs.tu-berlin.de
X230  - CAKx4u7kqvVH0-gstomsiVYdGC0i6=bGxzaQ8sq9gbg76TGme3w@mail.gmail.com
T420  - 9c848ee30b006737d0534d906bab0cf6@niklaas-baudet.net
T420s - 20120608080824.GS25324@hexapodia.org
W520  - 20121008181050.GF2549@ericlaptop.home.christensenplace.us

Signed-off-by: Richard Hartmann <richih.mailinglist@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>