HID: multitouch: add support of clickpads

Touchpads that have only one button are called clickpads and should
be advertised as such by the kernel.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>