Add support for fan button on Ideapad Z580

The patch adds support for fan control button on Ideapad Z580. This is
the same button as on Z570, but it raises different bit in
VPCCMD_R_SPECIAL_BUTTONS. Also add message to dmesg when unknown button
press detected, it will help adding support for new special buttons.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Tested-by: Ivan Vojtko <nekroman.sk@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>