Bluetooth: Add support for notifying userspace of new LTK's

If we want to have proper pairing support over LE we need to
inform userspace that a new LTK is available, so userspace
can store that key permanently.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>