Bluetooth: GATT: Add connection to bt_gatt_notify

This allow sending direct notification to a specific peer without
using CCC which is allowed by the spec:

  BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part F page 507
  3.4.7.1 Handle Value Notification

  A server can send a notification of an attribute’s value at any time.

Change-Id: Ieff29216cb9ba197c0da92d7b22b26e63101cfa8
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>