dm: sound: Create an option to use driver model for sound

The U-Boot sound system provides basic support for beeping. At present it
does not use driver model, but it needs to be converted. Add an option to
enable driver model for sound. For now it is not connected to anything.
Future work will add drivers which use this option. It will then be
removed once everything is converted.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index 947070f..35e2712 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -116,6 +116,7 @@
 	imply VIRTIO_SANDBOX
 	imply VIRTIO_BLK
 	imply VIRTIO_NET
+	imply DM_SOUND
 
 config SH
 	bool "SuperH architecture"