drm/i2c: tda998x: Register ASoC hdmi-codec and add audio DT binding

Register ASoC HDMI codec for audio functionality and adds device tree
binding for audio configuration.

With the registered HDMI codec the tda998x node can be used like a
regular codec node in ASoC card configurations. HDMI audio info-frame
and audio stream header is generated by the ASoC HDMI codec. The codec
also applies constraints for available sample-rates based on Edid Like
Data from the display. The device tree binding document has been
updated [1].

Part of this patch has been inspired by Jean Francoise's "drm/i2c: tda998x:
Add support of a DT graph of ports"-patch [2]. There may still be some
identical lines left from the original patch and some of the ideas
have come from there.

[1] Documentation/devicetree/bindings/display/bridge/tda998x.txt
[2] http://mailman.alsa-project.org/pipermail/alsa-devel/2015-July/095255.html

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

include/drm/i2c/tda998x.h

@@ -2,11 +2,12 @@
 #define __DRM_I2C_TDA998X_H__
 
 #include <linux/hdmi.h>
+#include <dt-bindings/display/tda998x.h>
 
 enum {
 	AFMT_UNUSED =	0,
-	AFMT_SPDIF =	1,
-	AFMT_I2S =	2,
+	AFMT_SPDIF =	TDA998x_SPDIF,
+	AFMT_I2S =	TDA998x_I2S,
 };
 
 struct tda998x_audio_params {