nfp: register a notifier handler in a central location for the device

Code interested in networking events registers its own notifier
handlers.  Create one device-wide notifier instance.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>