net: stmmac: xgmac: Add C45 PHY support in the MDIO callbacks

Add the support for C45 PHYs in the MDIO callbacks for XGMAC. This was
tested using Synopsys DesignWare XPCS.

v2:
- Pull out the readl_poll_timeout() calls into common code (Andrew)

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>