net: dsa: mv88e6xxx: 88e6390 10G serdes support

Add support for reading and reporting the 10G link status on the
88e6390 in addition to the 1000BASE-X/2500BASE-X/SGMII status.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>