Input: synaptics - add PNP ID for ThinkPad P50 to SMBus

Noticed the other day the trackpoint felt different on my P50, then
realized it was because rmi4 wasn't loading for this machine
automatically. Suspend/resume, hibernate, and everything else seem to
work perfectly fine on here.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>