mlxsw: spectrum_router: Add support for nexthop objects

Register a listener to the nexthop notification chain and parse notified
nexthop objects into the existing mlxsw nexthop data structures.

Signed-off-by: Ido Schimmel <[email protected]>
Reviewed-by: Jiri Pirko <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

Author: idosch