add elantec_touchscreen.idc from tf700 flavour

This is a pull-request which duplicates my [proposed changeset in gerrit](http://review.cyanogenmod.org/#/c/65566/) since it looks like this repo doesn't use CM's gerrit instance.

A CM11 install on the tf701t will almost work with the touchpad on the
keyboard dock except that, annoyingly, the touchpad is assumed to be an
actual touch screen device and so one ends up with the "circle"-style
mouse pointer.

Copying this file from the tf700 CM configuration results in the
expected "arrow"-style mouse pointer.