mlxsw: spectrum_router: Add GRE tunnel support for Spectrum-2

Spectrum-2 GRE tunnel implementation requires a specific underlay RIF that
points to the virtual router used for forwarding the encapsulated packet.

Add Spectrum-2 specific loopback router interface creation methods which
may create or reuse the dedicated underlay RIF.

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>