xtensa: implement dma_to_phys and phys_to_dma

This fixes the following build error seen in -next:

  drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:143:2: error:
  implicit declaration of function 'dma_to_phys'
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>

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

@@ -35,4 +35,14 @@ static inline struct dma_map_ops *get_dma_ops(struct device *dev)
 void dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 		    enum dma_data_direction direction);
 
+static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
+{
+	return (dma_addr_t)paddr;
+}
+
+static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t daddr)
+{
+	return (phys_addr_t)daddr;
+}
+
 #endif	/* _XTENSA_DMA_MAPPING_H */