powerpc: use <asm-generic/dma-mapping.h>

The powerpc version of dma-mapping.h only contains a version of
get_arch_dma_ops that always return NULL.  Replace it with the
asm-generic version that does the same.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190807150752.17894-1-hch@lst.de