net: add Hisilicon Network Subsystem MDIO support

The MDIO support for Hisilicon Network Subsystem. It is used in Hislicon
hip04, hip05 and Hi1610 SoC to control the external PHY

Signed-off-by: huangdaode <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: Kenneth Lee <liguozhu@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>