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

Conflicts:
	drivers/motor/immvibespi.c