sony-laptop: add AVMode key mapping

Some models are equipped with an "AVMode" function key that sends
  sony-laptop: Unknown event: 0x100 0xa1
  sony-laptop: Unknown event: 0x100 0x21
for press and release respectively.

Cc: "Matthew W. S. Bell" <matthew@bells23.org.uk>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Len Brown <len.brown@intel.com>