net: hns: add attribute port-rst-offset for dsaf port node

The reset offset for each port in a dsaf is different. The current code is
not so readability. This patch adds configuration named port-rst-offset to
make the code simple and more readability. If this attribute doesn't exist,
default value of this attribute is equal to its port index.

Signed-off-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>