mlxsw: spectrum: Add placeholder for dpipe

Add placeholder for dpipe. Support for specific tables and headers will
be introduced in following patches. The headers are shared between all
mlxsw_sp instances.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>