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/arm/mach-k3/sysfw-loader.c

@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <dm.h>
 #include <image.h>
 #include <log.h>
 #include <spl.h>