mlxsw: reg: Add underlay egress RIF field in RTDP register

In Spectrum-2 we need to specify the underlay egress router interface
when performing IP-in-IP and NVE packet decapsulation in the underlay
router.

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