use generic DMA mapping code in powerpc V4

[Christoph Hellwig]

Hi all,

this series switches the powerpc port to use the generic swiotlb and
noncoherent dma ops, and to use more generic code for the coherent
direct mapping, as well as removing a lot of dead code.

As this series is very large and depends on the dma-mapping tree I've
also published a git tree:

    git://git.infradead.org/users/hch/misc.git powerpc-dma.4

Gitweb:

    http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/powerpc-dma.4

Changes since v3:
 - rebase on the powerpc fixes tree
 - add a new patch to actually make the baseline amigaone config
   configure without warnings
 - only use ZONE_DMA for 64-bit embedded CPUs, on pseries an IOMMU is
   always present
 - fix compile in mem.c for one configuration
 - drop the full npu removal for now, will be resent separately
 - a few git bisection fixes

The changes since v1 are to big to list and v2 was not posted in public.