mlxsw: reg: Add Tunneling IPinIP Encapsulation ECN Mapping Register

This register performs mapping from overlay ECN to underlay ECN during
IPinIP encapsulation.

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