dm: stdio: Plumb in the new keyboard uclass

dm: stdio: Plumb in the new keyboard uclass

When driver model is used for keyboards we must scan the available keyboards
and register them with stdio. Add code to do this.

At some point (once LCD/video is converted) we should be able to convert
stdio to driver model and avoid these dual data structures.

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