net: phy: Add Synopsys DesignWare XPCS MDIO module

Synopsys DesignWare XPCS is an MMD that can manage link status,
auto-negotiation, link training, ...

In this commit we add basic support for XPCS using USXGMII interface and
Clause 73 Auto-negotiation.

This is highly tied with PHYLINK and can't be used without it. A given
ethernet driver can use the provided callbacks to add the support for
XPCS.

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