net: dsa: Add support for DSA specific notifiers

In preparation for communicating a given DSA network device's port
number and switch index, create a specialized DSA notifier and two
events: DSA_PORT_REGISTER and DSA_PORT_UNREGISTER that communicate: the
slave network device (slave_dev), port number and switch number in the
tree.

This will be later used for network device drivers like bcmsysport which
needs to cooperate with its DSA network devices to set-up queue mapping
and scheduling.

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