net: mscc: ocelot: Handle SWITCHDEV_PORT_ATTR_SET

Following patches will change the way we communicate setting a port's
attribute and use notifiers to perform those tasks.

Ocelot does not currently have an atomic notifier registered for
switchdev events, so we need to register one in order to deal with
atomic context SWITCHDEV_PORT_ATTR_SET events.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>