net: dsa: mv88e6xxx: add mv88e6321 rsvd2cpu

Add rsvd2cpu capability for mv88e6321 model, to allow proper bpdu
processing.

Signed-off-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Fixes: 51c901a7 ("net: dsa: mv88e6xxx: distinguish Global 2 Rsvd2CPU")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>