HID: multitouch: add support for trackpads

* some multitouch trackpads present the touch usage. This needs to be
filtered as it will conflict with mt-implementation.
* trackpads send BTN_TOOL_* to notify how many fingers are present
(this is used by xorg to use synaptics instead of generic evdev)
* trackpads like Perixx 701 are not different from a hid point of view
from a touchscreen, and we need to manually set them as touchpad.

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