net: hns3: add support for dumping MAC reg in debugfs

This patch adds support for dumping MAC reg in debugfs,
which will be helpful for debugging.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>