dm: core: Allow the uclass to set up a device's child after binding

For buses, after a child is bound, allow the uclass to perform some
processing. This can be used to figure out the address of the child (e.g.
the chip select for SPI slaves) so that it is ready to be probed.

This avoids bus drivers having to repeat the same process, which really
should be done by the uclass, since it is common.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>