Input: MT - handle semi-mt devices in core

Most semi-mt drivers use the slots in a manual way, but really only
need to treat the finger count manually. With this patch, a semi-mt
driver may use the input-mt core for everything else.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>