use generic DMA mapping code in powerpc V4

[Christoph Hellwig]

On Wed, Nov 28, 2018 at 10:05:19PM +1100, Michael Ellerman wrote:
> Is the plan that you take these via the dma-mapping tree or that they go
> via powerpc?

In principle either way is fine with me.  If it goes through the powerpc
tree we might run into a few minor conflicts with the dma-mapping tree
depending on how some of the current discussions go.