net: stmmac: enable clause 45 mdio support

DWMAC4 is capable to support clause 45 mdio communication.
This patch enable the feature on stmmac_mdio_write() and
stmmac_mdio_read() by following phy_write_mmd() and
phy_read_mmd() mdiobus read write implementation format.

Reviewed-by: Li, Yifan <yifan2.li@intel.com>
Signed-off-by: Kweh Hock Leong <hock.leong.kweh@intel.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>