HID: add n-trig digitizer usage

This adds a hid usage that is reported by the N-Trig digitizer in the Dell
Latitude XT screen.

Signed-off-by: Rafi Rubin <rafi@seas.upenn.edu>
Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>