net: mscc: ocelot: use separate flooding PGID for broadcast

In preparation of offloading the bridge port flags which have
independent settings for unknown multicast and for broadcast, we should
also start reserving one destination Port Group ID for the flooding of
broadcast packets, to allow configuring it individually.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>