dm: Add child_pre_probe() and child_post_remove() methods

dm: Add child_pre_probe() and child_post_remove() methods

Some devices (particularly bus devices) must track their children, knowing
when a new child is added so that it can be set up for communication on the
bus.

Add a child_pre_probe() method to provide this feature, and a corresponding
child_post_remove() method.

Signed-off-by: Simon Glass <sjg@chromium.org>