net: dsa: mv88e6xxx: Add SERDES phydev_mac_change up for 6390

phylink wants to know when the MAC layers notices a change in the
link. For the 6390 family, this is a change in the SERDES state.

Add interrupt support for the SERDES interface used to implement
SGMII/1000Base-X/2500Base-X. This is currently limited to ports 9 and
10. Support for the 10G SERDES and other ports will be added later,
building on this basic framework.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>