mtd: spi-mem: Drop dm.h header file

This header file should not be included in other header files. Remove it
and use a forward declaration instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/spi-mem.h

@@ -11,10 +11,7 @@
 #ifndef __UBOOT_SPI_MEM_H
 #define __UBOOT_SPI_MEM_H
 
-#include <common.h>
-#include <dm.h>
-#include <errno.h>
-#include <spi.h>
+struct udevice;
 
 #define SPI_MEM_OP_CMD(__opcode, __buswidth)			\
 	{							\