mlxsw: reg: Add fields to RITR - Router Interface Table Register

Add fields relevant for Spectrum-2 Loopback IPinIP router interface
creation. Add additional Loopback RIF protocol value - Generic, used for
creation of an explicit underlay RIF, and also add a field named
underlay_rif used for specifying the underlay RIF of a tunnel.

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