net: dsa: mv88e6xxx: mv88e6390X SERDES support

The mv88e6390X family has 8 SERDES lanes. These can be used for 2
10Gbps ports, ports 9 or 10. If these ports are used at slower speeds,
the SERDES lanes become available for other ports for 1000Base-X.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>