Input: synaptics - enable SMBus for HP EliteBook 840 G4

dmesg reports that "Your touchpad (PNP: SYN3052 SYN0100 SYN0002 PNP0f13)
says it can support a different bus."

I've tested the offered psmouse.synaptics_intertouch=1 with 4.18.x and
4.19.x and it seems to work well. No problems seen with suspend/resume.

Also, it appears that RMI/SMBus mode is actually required for 3-4 finger
multitouch gestures to work -- otherwise they are not reported at all.

Information from dmesg in both modes:

  psmouse serio3: synaptics: Touchpad model: 1, fw: 8.2, id: 0x1e2b1,
      caps: 0xf00123/0x840300/0x2e800/0x0, board id: 3139, fw id: 2000742

  psmouse serio3: synaptics: Trying to set up SMBus access
  rmi4_smbus 6-002c: registering SMbus-connected sensor
  rmi4_f01 rmi4-00.fn01: found RMI device,
      manufacturer: Synaptics, product: TM3139-001, fw id: 2000742

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