net: dsa: mv88e6xxx: add support for MV88E6220

The MV88E6220 is almost the same as MV88E6250 except that the ports 2-4 are
not routed to pins. So the usable ports are 0, 1, 5 and 6.

Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>