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 <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>