acer-wmi: support for P key on TM8372

commit 67e1d34cd54cbf33f093f1dd53e7bda1124eb972 upstream.

BugLink: http://launchpad.net/bugs/865807
There is no entry for P key on TM8372, so when P key is pressed, only
"acer_wmi: Unknown key number - 0x29" in dmesg.

Signed-off-by: Merlin Schumacher <merlin.schumacher@gmail.com>
Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>