mlxsw: spectrum: Introduce support for Data Center Bridging (DCB)

Introduce basic infrastructure for DCB and add the missing ops in
following patches.

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