ASoC: audio-graph-scu-card: tidyup "prefix" parsing on Doc

audio-graph-scu-card.c is supporting "prefix" which is used to avoid
DAI naming conflict when CPU/Codec matching.
But, sound card might have multi sub-devices, and each codec might need
each prefix.

Now, ASoC is supporting snd_soc_of_parse_node_prefix(), let's support
it on audio-graph-scu-card, too. It is keeping existing DT style.

It can't support each codec's prefix if sound card had multi sub-devices
without this patch.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/audio-graph-scu-card.txt

@@ -77,7 +77,6 @@ Example 2. 2 CPU 1 Codec (Mixing)
 		compatible = "audio-graph-scu-card";
 
 		label = "sound-card";
-		prefix = "codec";
 		routing = "codec Playback", "DAI0 Playback",
 			  "codec Playback", "DAI1 Playback",
 			  "DAI0 Capture",   "codec Capture";
@@ -90,6 +89,7 @@ Example 2. 2 CPU 1 Codec (Mixing)
 	audio-codec {
 		...
 
+		audio-graph-card,prefix = "codec";
 		port {
 			codec_endpoint0: endpoint {
 				remote-endpoint = <&cpu_endpoint0>;