macintosh: windfarm: use for_each_child_of_node() macro

Use for_each_child_of_node() macro instead of open coding it.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200914061411.3356-1-miaoqinglang@huawei.com