HID: wacom: generic: add 3 tablet touch keys

This patch add support to the 3 touch keys on Wacom Cintiq Pro.  These touch
keys are in the middle of the other two keys on the top edge of the tablet.

Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>