LoongArch: Add swiotlb backups buffer

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7AHL9

--------------------------------

Change-Id: I1b627be7ca7bd3690a175b3334c088e445278504
Signed-off-by: NJuxin Gao <gaojuxin@loongson.cn>
Signed-off-by: NHongchen Zhang <zhanghongchen@loongson.cn>

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

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2020 Loongson Technology Corporation Limited
+ */
+#ifndef _ASM_DMA_MAPPING_H
+#define _ASM_DMA_MAPPING_H
+
+extern const struct dma_map_ops loongson_dma_ops;
+extern bool swiotlb_need_fix;
+extern phys_addr_t io_tlb_start;
+
+static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
+{
+	if (swiotlb_need_fix)
+		return &loongson_dma_ops;
+	else
+		return NULL;
+}
+
+#endif /* _ASM_DMA_MAPPING_H */