HID: hid-multitouch: add support for the IDEACOM 6650 chip

The IDEACOM 6650 multitouch chip, present in various all-in-one computers,
uses the serial version of the HID multitouch protocol. No existing class
supports this.

In principle, the new MT_CLS_SERIAL should work for other
serial panels as well, perhaps including some eGalax panels.

Signed-off-by: Stephane Chatty <chatty@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>