drivers/nble: Implement notification for all conns

In a case connection is not known implement notification for all
connections the same way it is done for HCI stack. Attribute database
is scanned for CCC attribute and notifications are sent to all
connections.

Change-Id: Id556b4aa5af124aea672891ca89b278c71519643
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>