Sadly, the AUR packages have been very broken for some time. The easiest way to do it is just to install cabal-install and then install pandoc in your home directory with (as non-root):
cabal update cabal install pandoc Then you end up with pandoc in ~/.cabal/bin/pandoc.
It would be great if someone fixes the AUR packages, but until then, using cabal to install something in system risks making updates to ghc not work properly.
clarification
When you use cabal to install stuff in your home directory, it puts files under ~/.ghc and ~/.cabal. So worst case if you want to start fresh you can just recursively delete those directories. By contrast, when you install stuff in /usr without pacman (as would happen with cabal install --global ...), there might be residue there that makes it hard to upgrade ghc.