use the generic DMA direct remap code on powerpc

[Christoph Hellwig]

Hi powerpc maintainers,

this patch convers powerpc to use the generic dma remapping code
for the uncached coherent allocation on non-coherent CPUs.

Christophe Leroy tested a slightly earlier version on ppc8xx.

Note that I plan to move the need for the arch to call
dma_atomic_pool_init in this cycle, so either this needs to go in
through the dma-mapping tree (or a shared stable branch with it), or
we will need a small manual fixup in linux-next and when Linus merged
the later of the two pull requests.