immvibespi: add sysfs interface for controlling vibe intensity

This is based of codeworkx's work on smdk4412

Useful with something like this:
http://review.cyanogenmod.org/#/c/35981/

Change-Id: I1c1c16b4cefd600ebc6caa603d92044c466cf792
Merge remote-tracking branch 'upstream/cm-10.1' into HEAD