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/include/dm/ofnode.h b/include/dm/ofnode.h
index f608523..6414b46 100644
--- a/include/dm/ofnode.h
+++ b/include/dm/ofnode.h
@@ -501,6 +501,8 @@
 /**
  * ofnode_get_by_phandle() - get ofnode from phandle
  *
+ * This uses the default (control) device tree
+ *
  * @phandle:	phandle to look up
  * Return: ofnode reference to the phandle
  */