There isn't a field for burial site, but what you can do is use the cemetery category.
Just above the Biography section, there are edit buttons - one of them is for Categories. Click that and type the cemetery name (or part of it) into the search bar.
If the cemetery you need doesn't show up, it might not have a category created for it yet. You can then request the category be created. If you have any problems, come back here to G2G and post for assistance; make sure you use the "cemeteries" tag!
Hope this helps!!