Documentation: Add docs for max8925 dt

add docs for dt of max8925-mfd, max8925-backlight, and
max8925-battery

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>