Add support for the generic backlight interface below /sys/class/backlight.
Keep the procfs brightness handling for backward compatibility.

[apw@shadowen.org: backlight users need to select BACKLIGHT_CLASS_DEVICE]
Signed-off-by: NHolger Macht <hmacht@suse.de>
Signed-off-by: NAndy Whitcroft <apw@shadowen.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLen Brown <len.brown@intel.com>