sensors: Add dummy light sensor

Some devices have a sysfs toggle for the lightsensor instead of
passing the LUX values for userspace processing; for those,
use BOARD_SYSFS_LIGHT_SENSOR:=<sysfs path>

This is necessary for the rest of userspace to recognize its existence
and toggle it for auto-brightness. It doesn't generate actual light
values, though; all backlight adjustments are made in-kernel.

Change-Id: I0e546b4740720bd34d1e1d85a96b295fcc697106