fdt: Sync libfdt up to upstream

Add upstream changes to U-Boot:

- new pylibfdt functions
- fdt_setprop_placeholder()

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libfdt/fdt_rw.c b/lib/libfdt/fdt_rw.c
index 80a3212..3dc7752 100644
--- a/lib/libfdt/fdt_rw.c
+++ b/lib/libfdt/fdt_rw.c
@@ -228,8 +228,8 @@
 	return 0;
 }
 
-int fdt_setprop(void *fdt, int nodeoffset, const char *name,
-		const void *val, int len)
+int fdt_setprop_placeholder(void *fdt, int nodeoffset, const char *name,
+			    int len, void **prop_data)
 {
 	struct fdt_property *prop;
 	int err;
@@ -242,8 +242,22 @@
 	if (err)
 		return err;
 
+	*prop_data = prop->data;
+	return 0;
+}
+
+int fdt_setprop(void *fdt, int nodeoffset, const char *name,
+		const void *val, int len)
+{
+	void *prop_data;
+	int err;
+
+	err = fdt_setprop_placeholder(fdt, nodeoffset, name, len, &prop_data);
+	if (err)
+		return err;
+
 	if (len)
-		memcpy(prop->data, val, len);
+		memcpy(prop_data, val, len);
 	return 0;
 }