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>
diff --git a/drivers/fpga/socfpga_arria10.c b/drivers/fpga/socfpga_arria10.c
index dfd3cbb..44e1ac5 100644
--- a/drivers/fpga/socfpga_arria10.c
+++ b/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>