Phonet: back-end for autoconfigured addresses

In some cases, the network device driver knows what layer-3 address the
device should have. This adds support for the Phonet stack to
automatically request from the driver and add that address to the
network device.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>