dma-mapping: remove the dma_declare_coherent_memory export

dma_declare_coherent_memory is something that the platform setup code
(which pretty much means the device tree these days) need to do so that
drivers can use the memory as declared by the platform.  Drivers
themselves have no business calling this function.
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/coherent.c

@@ -122,7 +122,6 @@ int dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
 		dma_release_coherent_memory(mem);
 	return ret;
 }
-EXPORT_SYMBOL(dma_declare_coherent_memory);
 
 static void *__dma_alloc_from_coherent(struct dma_coherent_mem *mem,
 		ssize_t size, dma_addr_t *dma_handle)