mlxsw: reg: Add definition of unicast tunnel record for SFN register

Will be used to process learned FDB records from an NVE tunnel.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>