libfdt: change fdt.h to a wrapper of scripts/dtc/libfdt/*

Fortunately, U-Boot did not modify fdt.h locally.

Change it to a wrapper of scripts/dtc/libfdt/fdt.h, which will be
periodically synced with the upstream DTC (or kernel).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/include/fdt.h b/include/fdt.h
index f40b56c..b97b3f2 100644
--- a/include/fdt.h
+++ b/include/fdt.h
@@ -1 +1 @@
-#include "../lib/libfdt/fdt.h"
+#include "../scripts/dtc/libfdt/fdt.h"