HID: hid-multitouch: add support for PenMount dual-touch panel

This patch adds PenMount support to hid-multitouch.  A new class
MT_CLS_CONFIDENCE is defined for PenMount, since it uses HID_DG_CONFIDENCE as
the valid flag.

Signed-off-by: John Sung <penmount.touch@gmail.com>
[benjamin.tissoires@enac.fr: rebased on top of last_index_field changes]
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>