When I last looked into this, a year or two ago, you either had to roll your own core or license Xilinx's for beaucoup bucks.
Nowadays, I believe that Analog Devices may be supplying their own homebrew core. Have a surf through their Github repository and see what you can find. It would sure be nice if this is the case. Even if it's not licensed for use with non-AD parts, it would still provide a lot of useful clues for writing your own.