Input: elan_i2c - add trackstick report

The Elan touchpads over I2C/SMBus also can handle a trackstick.
Unfortunately, nothing tells us if the device supports trackstick (the
information lies in the PS/2 node), so rely on device properties to
determine whether to enable the trackstick node.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1313939



Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: KT Liao <kt.liao@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>