powerpc/dma: trim the fat from <asm/dma-mapping.h>

There is no need to provide anything but get_arch_dma_ops to
<linux/dma-mapping.h>.  More the remaining declarations to <asm/iommu.h>
and drop all the includes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>