mtd: ofpart: use for_each_child_of_node() macro

Use for_each_child_of_node() macro instead of open coding it.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>