pm8xxx-vibrator: exposed sysfs to modify voltage level

should show up as /sys/devices/platform/msm_ssbi.0/pm8921-core/pm8xxx-vib/amp

there are only 20 discrete steps though (12 to 31)
shizz

Change-Id: I472b9d39f00898c6c67dd7b99ac3118bb70cf292