dma-mapping: add <asm/dma-mapping.h> for all architectures

To avoid "asm/dma-mapping.h: No such file or directory" error,
we need something.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/sandbox/include/asm/dma-mapping.h b/arch/sandbox/include/asm/dma-mapping.h
new file mode 100644
index 0000000..853b087
--- /dev/null
+++ b/arch/sandbox/include/asm/dma-mapping.h
@@ -0,0 +1 @@
+/* SPDX-License-Identifier: GPL-2.0-only */