Input: synaptics-rmi4 - forward upper mechanical buttons to PS/2 guest

On the latest series of ThinkPads, the button events for the TrackPoint
are reported through the touchpad itself as opposed to the TrackPoint
device. In order to report these buttons properly, we need to forward
them to the TrackPoint device and notify psmouse to send the button
presses/releases.

Signed-off-by: Lyude Paul <thatslyude@gmail.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>