mlxsw: reg: Add Management DownStream Device Tunneling Register

The MDDT register allows to deliver query and request messages (PRM
registers, commands) to a DownStream device.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>