touchpad: Keep track of associated trackpoint device

touchpad: Keep track of associated trackpoint device

The top soft buttons are intended for use with a trackpoint, and to e.g.
make middle button scrolling work correctly, we must post the events for
these "buttons" through the trackpoint device.

This commit is a preparation patch for this, it adds a link to the
trackpoint to the touchpad, but does not yet do anything with it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>