Input: Add INPUT_PROP_TOPBUTTONPAD device property

On some newer laptops with a trackpoint the physical buttons for the
trackpoint have been removed to allow for a larger touchpad. On these
laptops the buttonpad has clearly marked areas on the top which are to be
used as trackpad buttons.

Users of the event device-node need to know about this, so that they can
properly interpret BTN_LEFT events as being a left / right / middle click
depending on where on the button pad the clicking finger is.

This commits adds a INPUT_PROP_TOPBUTTONPAD device property which drivers
for such buttonpads will use to signal to the user that this buttonpad not
only has the normal bottom button area, but also a top button area.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>