powerpc/iommu: Support "hybrid" iommu/direct DMA ops for coherent_mask < dma_mask

This patch adds the ability to the DMA direct ops to fallback to the IOMMU
ops for coherent alloc/free if the coherent mask of the device isn't
suitable for accessing the direct DMA space and the device also happens
to have an active IOMMU table.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --cc arch/powerpc/Kconfig
Simple merge

diff --cc arch/powerpc/include/asm/dma-mapping.h
Simple merge

diff --cc arch/powerpc/kernel/dma-iommu.c
Simple merge

diff --cc arch/powerpc/kernel/dma-swiotlb.c
Simple merge

diff --cc arch/powerpc/kernel/dma.c
Simple merge