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>
diff --git a/test/dm/sound.c b/test/dm/sound.c
index 3767abb..aa5368f 100644
--- a/test/dm/sound.c
+++ b/test/dm/sound.c
@@ -28,6 +28,7 @@
 	ut_asserteq(4560, sandbox_get_sound_sum(dev));
 	ut_assertok(sound_beep(dev, 1, 100));
 	ut_asserteq(9120, sandbox_get_sound_sum(dev));
+	ut_asserteq(false, sandbox_get_sound_active(dev));
 
 	return 0;
 }