Implement bus dma support for loading raw mappings so that we can use

Implement bus dma support for loading raw mappings so that we can use
xhci(4) on ARM.  The only way the load raw operation can get to know
about the coherent flag is via the segments.  Store it there when the
memory is initially mapped.  Also store the virtual address which we
need to know when we have to flush the caches on a non-coherent mapping.

ok kettenis@