mlxsw: spectrum_buffers: Push out indexes and direction out of SB structs

Structs are in arrays so use array index as pool/tc/prio index. With
that, there is need to maintain separate arrays for ingress and egress.
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>