Input: synaptics - add support for Intertouch devices

Most of the Synaptics devices are connected through PS/2 and a different
bus (SMBus or HID over I2C). The secondary bus capability is indicated by
the InterTouch bit in extended capability 0x0C.

We only enable the InterTouch device to be created for the laptops
registered with the top software button property or those we know that are
functional. In the future, we might change the default to always rely on
the InterTouch bus. Currently, users can enable/disable the feature with
the psmouse parameter synaptics_intertouch.

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