charger: Toggle backlight while in charger mode.

The backlight is now toggled on/off in the charger mode via a sysfs
node. This is to compensate for the fact that on certain targets the
FBIOBLANK ioctl does not turn off the backlight.

Change-Id: I7c4531e7f1b0599b9a1f5ed6f95d13ee6a664066
CRs-fixed: 514940