Add QBluetoothDeviceInfo::UnknownCoreConfiguration enum value

The value is needed for platforms which do not provide this type
information. An example are Android versions below 18.

The enum was added in Qt 5.4. Therefore we need to add
the missing \since tag too.

Change-Id: I23582e7500d8da6740b281ba9821d028c4f33e05
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>