sound: Add a new stop_play() method

At present there is no positive indication that U-Boot has finished
sending sound data. This means that it is not possible to power down an
audio codec, for example. Add a new method that is called once all sound
data has been sent.

Add a new method for this, called when the sound_play() call is done.

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