mlxsw: spectrum_trap: Add devlink-trap policer support

Register supported packet trap policers with devlink and implement
callbacks to change their parameters and read their counters.

Prevent user space from passing invalid policer parameters down to the
device by checking their validity and communicating the failure via an
appropriate extack message.

v2:
* Remove the max/min validity checks from __mlxsw_sp_trap_policer_set()

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