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/of_access.h b/include/dm/of_access.h
index 83f34f0..d8c6d11 100644
--- a/include/dm/of_access.h
+++ b/include/dm/of_access.h
@@ -258,11 +258,13 @@
 /**
  * of_find_node_by_phandle() - Find a node given a phandle
  *
+ * @root:	root node to start from (NULL for default device tree)
  * @handle:	phandle of the node to find
  *
  * Return: node pointer, or NULL if not found
  */
-struct device_node *of_find_node_by_phandle(phandle handle);
+struct device_node *of_find_node_by_phandle(struct device_node *root,
+					    phandle handle);
 
 /**
  * of_read_u8() - Find and read a 8-bit integer from a property