net: dsa: mv88e6xxx: Add SERDES stats counters to all 6390 family members

The SERDES statistics are valid for all members of the 6390 family,
not just the 6390 itself. Add the needed callbacks to all members of
the family.

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