dma-mapping: provide a generic asm/dma-mapping.h

For architectures that just use the generic dma_noop_ops we can provide
a generic version of dma-mapping.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 970460a..197460c 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -7,6 +7,7 @@
 generic-y += div64.h
 generic-y += dma.h
 generic-y += dma-contiguous.h
+generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += errno.h
 generic-y += exec.h