swiotlb-xen cleanups v2

From: Christoph Hellwig
Date: Mon Aug 26 2019 - 08:19:50 EST

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