nds32: DMA mapping API

This patch adds support for the DMA mapping API. It uses dma_map_ops for
flexibility.
Signed-off-by: Vincent Chen <vincentc@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

arch/nds32/include/asm/dma-mapping.h

+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2005-2017 Andes Technology Corporation
+
+#ifndef ASMNDS32_DMA_MAPPING_H
+#define ASMNDS32_DMA_MAPPING_H
+
+extern struct dma_map_ops nds32_dma_ops;
+
+static inline struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
+{
+	return &nds32_dma_ops;
+}
+
+#endif