mlxsw: core: Introduce flexible actions support

Each entry which is matched during ACL lookup points to an action set.
This action set contains up to three separate actions. If more actions
are needed to be chained, the extended set is created to hold them
in KVD linear area.

This patch implements handling of sets and encoding of actions.
Currectly, only two actions are supported. Drop and forward. Forward
action uses PBS pointer to KVD linear area, so the action code needs to
take care of this as well.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>