net: dsa: qca8k: add support for mdb_add/del

Add support for mdb add/del function. The ARL table is used to insert
the rule. The rule will be searched, deleted and reinserted with the
port mask updated. The function will check if the rule has to be updated
or insert directly with no deletion of the old rule.
If every port is removed from the port mask, the rule is removed.
The rule is set STATIC in the ARL table (aka it doesn't age) to not be
flushed by fast age function.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>