dm: core: Allow uclass to set up a device's child after it is probed

Some buses need to set up their child devices after they are probed.
Support a common child_post_probe() method for the uclass.

With this change, the two APIs uclass_pre_probe_device() and
uclass_post_probe_device() become symmetric.

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