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-stm32mp/cmd_stm32prog/stm32prog_serial.c

@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <console.h>
+#include <dm.h>
 #include <dfu.h>
 #include <malloc.h>
 #include <serial.h>