dm: core: Pass a root node to of_find_node_by_phandle()

This function currently assumes that the control FDT is used. Update it
to allow a root node to be passed, so it can work with any tree.

Also add a comment to ofnode_get_by_phandle() so that its purpose is
clear.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/ofnode.c b/drivers/core/ofnode.c
index 42f3c09..b241be3 100644
--- a/drivers/core/ofnode.c
+++ b/drivers/core/ofnode.c
@@ -391,7 +391,7 @@
 	ofnode node;
 
 	if (of_live_active())
-		node = np_to_ofnode(of_find_node_by_phandle(phandle));
+		node = np_to_ofnode(of_find_node_by_phandle(NULL, phandle));
 	else
 		node.of_offset = fdt_node_offset_by_phandle(gd->fdt_blob,
 							    phandle);