switchdev: introduce switchdev notifier

This patch introduces new notifier for purposes of exposing events which happen
on switch driver side. The consumers of the event messages are mainly involved
masters, namely bridge and ovs.

Suggested-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>