net: dsa: mv88e6xxx: Add missing watchdog ops for 6320 family

[ Upstream commit 9c7f37e5ca14f5b04894b1b699a9903885cdafa6 ]

The 6320 family of switches uses the same watchdog registers as the
6390.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 6686317855c6 ("net: dsa: mv88e6xxx: add mv88e6321 rsvd2cpu")
Signed-off-by: Sasha Levin <sashal@kernel.org>