mlxsw: spectrum_buffers: Configure MC pool

Pool 15 (indexed as 8) is dedicated to MC traffic. Its configuration has
been kept at default, because the table-based configuration wasn't
expressive enough to allow the explicit configuration.

Now that the configuration of pool 15 can be described, do so. The MC
pool should have infinite size, infinite per-TC quota, and per-port
limit of 90K.

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