dm: core: improve uclass_get_device_by_phandle_id() description

Complete the devp parameter description.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/uclass.h b/include/dm/uclass.h
index 67ff746..7188304 100644
--- a/include/dm/uclass.h
+++ b/include/dm/uclass.h
@@ -224,7 +224,8 @@
  *
  * @id: uclass ID to look up
  * @phandle_id: the phandle id to look up
- * @devp: Returns pointer to device (there is only one for each node)
+ * @devp: Returns pointer to device (there is only one for each node). NULL if
+ *	there is no such device.
  * @return 0 if OK, -ENODEV if there is no device match the phandle, other
  *	-ve on error
  */