input: Add keyreset driver.

Add a platform device in the board file to specify a reset key-combo.
The first time the key-combo is detected a work function that syncs
the filesystems is scheduled. If all the keys are released and then
pressed again, it calls panic. Reboot on panic should be set for
this to work.

Change-Id: I9d54283ca1fba45e4b1ae1a407524cdda8171143
Signed-off-by: Arve Hjønnevåg <arve@android.com>