net: dsa: b53: Add support for MDB

In preparation for supporting IGMP snooping with or without the use of
a bridge, add support within b53_common.c to program the ARL entries for
multicast operations. The key difference is that a multicast ARL entry
is comprised of a bitmask of enabled ports, instead of a port number.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>