mlxsw: spectrum: Expose functions to create and destroy underlay RIF

In Spectrum-2, instead of providing the ID of the virtual router (VR)
where NVE underlay lookups will occur as in Spectrum-1, the ID of a
router interface (RIF) in this VR is required.

Expose functions to create and destroy such a RIF.

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