mlxsw: spectrum: Add policers for trap groups

Configure policers and connect them to trap groups.

While many trap groups share policer's configuration they don't share
the actual policer because each trap group represents a different
flow / protocol and we don't want one of them to be able to exceed its
rate on behalf of another.
For example, if STP and LLDP gets to send 128 packets/sec each, if we
put them in one 256 packets/sec policer, one can send 200 packets while
the other only 50.

Note that IP2ME covers lots of flows, so it's limit is set to match the
cpu ability to handle data.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>