Input: elantech - report multitouch with proper ABS_MT messages
Multitouch info was reported only via a old protocol used by the
proprietary X driver from elantech. Let's report the multitouch info
also following the official MT protocol. It's semi-mt because the device
only reports the lowest/highest coordinates.
This was done following the multi-touch-protocol.txt documentation, and
inspired by the bcm5974 and elantech implementations. Testing was light
as there is not many applications using this protocol yet, but the X
synaptics driver didn't complain and the X multitouch driver behaved
correctly.