mlxsw: reg: Add Shared Buffer Internal Buffer register

The SBIB register configures per port buffer for internal use. This
register is used to configure an egress mirror buffer on the egress port
which does the mirroring.

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