dm: uclass: Adding missing child_pre_probe description

The comment of child_pre_probe, one of the 'struct uclass_driver'
members, is currently missing.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/dm/uclass.h

@@ -58,6 +58,7 @@ struct udevice;
  * @post_probe: Called after a new device is probed
  * @pre_remove: Called before a device is removed
  * @child_post_bind: Called after a child is bound to a device in this uclass
+ * @child_pre_probe: Called before a child is probed in this uclass
  * @init: Called to set up the uclass
  * @destroy: Called to destroy the uclass
  * @priv_auto_alloc_size: If non-zero this is the size of the private data