Add support for the trackpoint and three mouse buttons on the type cover
of the Lenovo X1 Tablet Gen3.

This is the same as with the 2nd generation Lenovo X1 Tablet.
Signed-off-by: Mikael Wikström <leakim.wikstrom@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>