dsa: mv88e6352/mv88e6xxx: Add support for Marvell 88E6320 and 88E6321

MV88E6320 and MV88E6321 are largely compatible to MV886352,
but are members of a different chip family.

Signed-off-by: Aleksey S. Kazantsev <ioctl@yandex.ru>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>