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

Author: Christoph Hellwig

arch/powerpc/include/asm/Kbuild

@@ -4,6 +4,7 @@ generated-y += syscall_table_64.h
 generated-y += syscall_table_c32.h
 generated-y += syscall_table_spu.h
 generic-y += div64.h
+generic-y += dma-mapping.h
 generic-y += export.h
 generic-y += irq_regs.h
 generic-y += local64.h