of_numa: Use of_get_next_parent to simplify code

Use of_get_next_parent() instead of open-code.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>