I use the crop tool with perspective box checked on my desktop, would like to use it on my iPad.
I thought that was probably the closest thing, but I rarely use the skew tools in my CS photoshop, so I don't know how it works. I'll have to go to the tutorials section on this site to learn
Another option would be the Warp option under the Amperstand "&" menu. You would want to move the outer frame handles out from the vertical center. It it's not possible to link them, so you would need to move each side individually and crop as an additional step afterwards.
(click images to view larger)
probably would need to move the top right frame handle out event a little more here
Europe, Middle East and Africa