swiotlb-xen cleanups v2

[Christoph Hellwig]

Hi Xen maintainers and friends,

please take a look at this series that cleans up the parts of swiotlb-xen
that deal with non-coherent caches.

Changes since v1:
 - rewrite dma_cache_maint to be much simpler
 - improve various comments and commit logs
 - remove page-coherent.h entirely