Add Elantech trackpad support

Elantech trackpads are found in some laptops like the Asus UX31E. They
are "synaptics compatible" but use a slightly different protocol.

Elantech hardware support is not enabled by default and just like
Synaptic or TrackPoint devices it should be enabled by setting
tunable, in this case hw.psm.elantech_support, to non-zero value

PR: 205690
Submitted by: Vladimir Kondratyev <wulf@cicgroup.ru>
MFC after: 1 week