mlxsw: spectrum: Add devlink-trap support

Register supported packet traps (layer 2 drops only, currently) and
associated trap group with devlink during driver initialization.

The amount of traffic generated by these packet drop traps is capped at
10Kpps to ensure the CPU is not overwhelmed by incoming packets.

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