net: dsa: mv88e6xxx: 6390 vs 6390X SERDES support

The 6390 has two SERDES interfaces, used by ports 9 and 10.  The 6390X
has eight SERDES interfaces. These allow ports 9 and 10 to do 10G. Or
if lower speeds are used, some of the SERDES interfaces can be used by
ports 2-8 for 1000Base-X.

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