fs: fs-loader: 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>

drivers/fpga/socfpga_arria10.c

@@ -13,6 +13,7 @@
 #include <altera.h>
 #include <asm/arch/pinmux.h>
 #include <common.h>
+#include <dm.h>
 #include <dm/ofnode.h>
 #include <errno.h>
 #include <fs_loader.h>