pm8xxx-vibrator: implement sysfs for intensity level

Node: /sys/devices/virtual/timed_output/vibrator/level
Range: 12 - 31

Change-Id: I7c2d390cb55c9d39ad9a8cc7e6eb7687832d4465
Merge pull request #1 from CyanogenMod/cm-11.0

pm8xxx-vibrator: implement sysfs for intensity level