sf: Drop dm.h header file from spi_flash.h

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>

arch/x86/cpu/ivybridge/sdram.c

@@ -10,6 +10,7 @@
  */
 
 #include <common.h>
+#include <dm.h>
 #include <errno.h>
 #include <fdtdec.h>
 #include <init.h>