sony-laptop: Add support for extra keyboard events

The current sony-laptop code assumes that the keyboard event method is
always located at slot 2 in the platform code. Remove this assumption and
add support for some additional hotkeys.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Len Brown <len.brown@intel.com>