net: hns3: Add MDIO support to HNS3 Ethernet driver for hip08 SoC

This patch adds the support of MDIO bus interface for HNS3 driver.
Code provides various interfaces to start and stop the PHY layer
and to read and write the MDIO bus or PHY.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: lipeng <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>