dm: core: Split ofnode_path_root() into two functions

This function turns out to be a little confusing since it looks up a path
and also registers the tree. Split it into two, one that gets the root
node and one that looks up a path, so the purpose is clear.

Registering the tree will happen in a function to be added in a later
patch, called oftree_from_fdt().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/vbe_simple.c b/boot/vbe_simple.c
index 0fc5738..61b6322 100644
--- a/boot/vbe_simple.c
+++ b/boot/vbe_simple.c
@@ -240,7 +240,7 @@
 			continue;
 
 		/* Check if there is a node to fix up */
-		node = ofnode_path_root(tree, "/chosen/fwupd");
+		node = oftree_path(tree, "/chosen/fwupd");
 		if (!ofnode_valid(node))
 			continue;
 		node = ofnode_find_subnode(node, dev->name);