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>

include/dm/ofnode.h

@@ -501,6 +501,8 @@ int ofnode_get_path(ofnode node, char *buf, int buflen);
 /**
  * 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
  */