devlink: Add layer 2 control packet traps

Add layer 2 control packet traps such as STP and IGMP query, so that
capable device drivers could register them with devlink. Add
documentation for every added packet trap and packet trap group.

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