mlxsw: spectrum_buffers: Add a multicast pool for Spectrum-2

In Spectrum-1, when a multicast packet is admitted to the shared buffer
it increases the quotas of all the ports and {port, TC} to which it is
forwarded to.

The above means that multicast packets are accounted multiple times in
the shared buffer and can therefore cause the associated shared buffer
pool to fill up very quickly.

To work around this issue, commit e83c045e ("mlxsw:
spectrum_buffers: Configure MC pool") added a dedicated multicast pool
in which multicast packets are accounted.

The issue is not present in Spectrum-2, but in order to be backward
compatible with Spectrum-1, its default behavior is to allow a multicast
packet to increase multiple egress quotas instead of one.

Until the new (non-backward compatible) mode is supported, configure a
dedicated multicast pool as in Spectrum-1.

Fixes: fe099bf6 ("mlxsw: spectrum_buffers: Add Spectrum-2 shared buffer configuration")
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>