mlxsw: spectrum: Add struct mlxsw_sp_sb_vals

Spectrum-2 will be configured with a different shared buffer
configuration than Spectrum-1. Therefore introduce a structure for
keeping the chip-specific default and immutable configuration.

Configuration mutable in runtime will still be kept in struct
mlxsw_sp_sb.

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