HID: Multitouch support for the N-Trig touchscreen

Adds support for multitouch interaction on the N-Trig touchscreen, using the
new ABS_MT_* input constants. Single touch support works as previously. This
code was tested against two versions of the N- Trig firmware: one that supports
dual pen/finger single touch, and one that supports finger multitouch but no
pen at all. Copyright notices that looked wrong were removed, as it seems that
there is only code written in 2009 by Rafin Rubin and Stephane Chatty in this
file.

Signed-off-by: Stephane Chatty <chatty@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>