HID: multitouch: support buttons and trackpoint on Lenovo X1 Tab Gen2

On the 2nd generation Lenovo Tablet only clickpad is working; the
trackpoint and three mouse buttons do not work.

hid_multitouch must export all inputs in order to get trackpoint and
buttons to function.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>