mlxsw: spectrum_buffers: Implement shared buffer configuration

Implement previously introduced mlxsw core shared buffer API.
For Spectrum, that is done utilizing registers SBPR, SBCM and SBPM.

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