Input: wacom - add touch sensor support for Cintiq 24HD touch

Decode multitouch reports from the touch sensor of the Cintiq 24HD
touch.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>