aat2870: mod auto brightness and add sysfs interface

Change-Id: I4103dd526c1a4276a2ac5ca07a156b860493919f
Signed-off-by: nicholas <firepoison35@gmail.com>

Conflicts:
	drivers/video/backlight/aat2870.c
Merge branch 'jb-412' of https://github.com/MinoochX/lge-kernel-star into jb-412