drivers/video/backlight/as3711_bl.c: add OF support

Add support for configuring AS3711 backlight driver from DT.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>