mlxsw: reg: Add Routing Tunnel Decap Properties Register

The RTDP register is used for configuring the tunnel decap properties of
NVE and IPinIP.

Signed-off-by: Petr Machata <petrm@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>