Not all devices use the same keycodes in the same order,
so add possibility to define keycodes for buttons present
on actual hardware.

If keycodes property is not present, we assume that device has
at least MENU and BACK keys.
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>