Bluetooth: Introduce HCI_CONN_FLAG_DEVICE_PRIVACY device flag

This introduces HCI_CONN_FLAG_DEVICE_PRIVACY which can be used by
userspace to indicate to the controller to use Device Privacy Mode to a
specific device.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>