Input: synaptics - gate forcepad support by DMI check

Unfortunately, ForcePad capability is not actually exported over PS/2, so
we have to resort to DMI checks.

Cc: stable@vger.kernel.org
Reported-by: Nicole Faerber <nicole.faerber@kernelconcepts.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>