dm: sound: Create a uclass for i2s

The i2s bus is commonly used with audio codecs. It provides a way to
stream digital data sychronously in both directions. U-Boot only supports
audio output, so this uclass is very simple, with a single tx_data()
method.

Add a uclass and a test for i2s.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/sound/Makefile b/drivers/sound/Makefile
index ae5fabe..4aced9d 100644
--- a/drivers/sound/Makefile
+++ b/drivers/sound/Makefile
@@ -5,6 +5,7 @@
 
 obj-$(CONFIG_SOUND)	+= sound.o
 obj-$(CONFIG_DM_SOUND)	+= codec-uclass.o
+obj-$(CONFIG_DM_SOUND)	+= i2s-uclass.o
 obj-$(CONFIG_I2S)	+= sound-i2s.o
 obj-$(CONFIG_I2S_SAMSUNG)	+= samsung-i2s.o
 obj-$(CONFIG_SOUND_SANDBOX)	+= sandbox.o