Drivers are supposed to be able to close down cleanly. To set a good example,
make sandbox shut down its driver model drivers and remove them before exit.

It may be desirable to do the same more generally once driver model is more
widely-used. This could be done during bootm, before U-Boot jumps to the OS.
It seems far too early to make this change.

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