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>

board/st/common/stm32mp_dfu.c

@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <blk.h>
+#include <dm.h>
 #include <dfu.h>
 #include <env.h>
 #include <memalign.h>