The Batman: Arkham Knight PlayStation bonus DLC is a timed exclusive.
The bonus content those buying Batman: Arkham Knight on PlayStation 4 will get, may end up coming to other platforms. As is usually the case with these types of DLC exclusivity deals, there's always a duration.
As you can see, the content appears to be exclusive until at least fall 2015. A mere few months away from game's June release date.
Warner has not made any announcements about if/when this content will come to other platforms.