mlxsw: spectrum_buffers: Use devlink pool indices throughout

Currently, mlxsw assumes that each ingress pool has its egress
counterpart, and that pool index for purposes of caching matches the
index with which the hardware should be configured. As we want to expose
the MC pool, both of these assumptions break.

Instead, maintain the pool index as long as possible. Unify ingress and
egress caches and use the pool index as cache index as well. Only
translate to FW pool numbering when actually packing the registers. This
simplifies things considerably, as the pool index is the only quantity
necessary to uniquely identify a pool, and the pool/direction split is
not necessary until firmware is talked to.

To support the mapping between pool indices and pool numbers and
directions, which is not neatly mathematical anymore, introduce a pool
descriptor table, indexed by pool index, to facilitate the translation.

Include the MC pool in the descriptor table as well, so that it can be
referenced from mlxsw_sp_sb_cms_egress.
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>