mlxsw: spectrum_buffers: Add pools for CPU traffic

Packets that are trapped to the CPU are transmitted through the CPU port
to the attached host. The CPU port is therefore like any other port and
needs to have shared buffer configuration.

The maximum quotas configured for the CPU are provided using dynamic
threshold and cannot be changed by the user. In order to make sure that
these thresholds are always valid, the configuration of the threshold
type of these pools is forbidden.

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