mfd: max8925: Support dt for backlight

Add device tree support in max8925 backlight.

Signed-off-by: Qing Xu <qingx@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>