mlxsw: Add trap group for layer 2 discards

Discard trap groups are defined in a different enum so that they could
all share the same policer ID: MLXSW_REG_HTGT_TRAP_GROUP_MAX + 1.

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