provide in-place uncached remapping for dma-direct v2

[Christoph Hellwig]

Hi all,

this series provides support for remapping places uncached in-place in
the generic dma-direct code, and moves openrisc over from its own
in-place remapping scheme.  The arm64 folks also had interest in such
a scheme to avoid problems with speculating into cache aliases.

Also all architectures that always use small page mappings for the
kernel and have non-coherent DMA should look into enabling this
scheme, as it is much more efficient than the vmap remapping.

Changes since v1:
 - share the arch hook for inline remap and uncached segment support