macintosh: Use for_each_child_of_node() macro

Use for_each_child_of_node() macro instead of open coding it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1623034908-30525-1-git-send-email-zou_wei@huawei.com