Input: add keycodes for touchpad on/off keys

Some laptops will have a "touchpad toggle" soft button, which expects
user-space to turn off the touchpad themselves, some other devices will
do this in hardware, but send key events telling us that the touchpad
has been turned off/on.

KEY_TOUCHPAD_ON/KEY_TOUCHPAD_OFF will be used by user-space to show a
popup with the status of the touchpad.

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>