nds32: introduce DMA allocation API

U-Boot does not compile for the adp-ag101 boards since
commit a8f9cd1893
(net: update FTGMAC100 for MMU/D-cache support)

The driver assumes that the DMA allocation API are provided by all
architectures. This is not the case for nds32 and it causes a
build error. This patch adds DMA allocation API to avoid the errors.

Signed-off-by: Kuan-Yu Kuo <ken.kuoky@gmail.com>
Cc: Macpaul Lin <macpaul@gmail.com>
Cc: Andes <uboot@andestech.com>
Signed-off-by: Andes <uboot@andestech.com>

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2013 Andes Technology Corporation
+ * Ken Kuo, Andes Technology Corporation <ken_kuo@andestech.com>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+#ifndef __ASM_NDS_DMA_MAPPING_H
+#define __ASM_NDS_DMA_MAPPING_H
+
+enum dma_data_direction {
+	DMA_BIDIRECTIONAL	= 0,
+	DMA_TO_DEVICE		= 1,
+	DMA_FROM_DEVICE		= 2,
+};
+
+static void *dma_alloc_coherent(size_t len, unsigned long *handle)
+{
+	*handle = (unsigned long)memalign(ARCH_DMA_MINALIGN, len);
+	return (void *)*handle;
+}
+
+static inline unsigned long dma_map_single(volatile void *vaddr, size_t len,
+					   enum dma_data_direction dir)
+{
+	return (unsigned long)vaddr;
+}
+
+static inline void dma_unmap_single(volatile void *vaddr, size_t len,
+				    unsigned long paddr)
+{
+}
+
+#endif /* __ASM_NDS_DMA_MAPPING_H */