mlxsw: spectrum_router: Add a dedicated notifier block

Currently all netdevice events are handled in the centralized notifier
handler maintained by spectrum.c. Since a number of events are involving
router code, spectrum.c needs to dispatch them to spectrum_router.c. The
spectrum module therefore needs to know more about the router code than it
should have, and there is are several API points through which the two
modules communicate.

To simplify the notifier handlers, introduce a new notifier into the router
module.

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