HID: multitouch: support Perixx PERIPAD 701

Perixx Peripad 701 is an hybrid device which presents a touchpad and
a keyboard on the same surface. The switch between the two is controlled
by a physical switch, and the firmware sends the events on the right
interface (mouse, keyboard or multitouch).
This patch enables the multitouch interface of this device to work.

We need to manually set the device as a trackpad (we cannot infer it
from the reports descriptors as the device works under Windows, a system
that does not allow multitouch touchpad).
We also need to set the hid feature MAX CONTACT NUMBER to 2 or the device
stops sending events once it has been pressed by two touches.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>