mlxsw: spectrum_trap: Introduce dummy group with thin policer

For "source traps" it is not possible to change HPKT action to discard.
But there is still need to disallow packets arriving to CPU as much as
possible. Handle this by introduction of a "dummy group". It has a
"thin" policer, which passes as less packets to CPU as possible. The
rest is going to be discarded there. The "dummy group" is to be used
later on by ACL trap (which is a "source trap").

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