net: mgmt: Add IPv6 DAD succeed/failed event

We need a way to know when IPv6 address is successfully set
into network interface.

If IPv6 DAD (Duplicate Address Detection) succeeds or fails,
we send a management event for that. This can be used by
other components to detect when the network interface is in
usable state.

Change-Id: Ifb22415fe21f31f5dba4f55455d6e0f89b414d32
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>