dt-bindings: net: dsa: marvell: add 6220 model to the 6250 family

The MV88E6220 is part of the MV88E6250 family.

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>