Bluetooth: Add support for handling P-256 derived link keys

Before being able to enable Secure Connections support, the core needs
to know on how to handle P-256 derived link keys. The difference between
authenticated and unauthenticated P-256 derived link keys is the same as
its P-192 counter parts.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>