net: dsa: mv88e6xxx: add port mask helper

Add a mv88e6xxx_port_mask() helper to get the bitmask of ports in a
switch chip, that will be used in several features.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>