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

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>