mlxsw: spectrum_router: Enable local routes promotion to perform NVE decap

When an NVE tunnel with an IP underlay (e.g., VxLAN) is configured the
local route to the tunnel's source IP needs to be promoted to perform
NVE decapsulation.

Expose an API in the unicast IP router to promote / demote local routes.

The case where a local route is configured after the creation of the NVE
tunnel will be handled in a subsequent patch in the set.

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