ASoC: simple-scu-card: tidyup "prefix" parsing on Doc

simple-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/simple-scu-card.txt

@@ -75,7 +75,6 @@ sound {
 	simple-audio-card,bitclock-master = <&dpcmcpu>;
 	simple-audio-card,frame-master = <&dpcmcpu>;
 
-	simple-audio-card,prefix = "ak4642";
 	simple-audio-card,routing = "ak4642 Playback", "DAI0 Playback",
 			"ak4642 Playback", "DAI1 Playback";
 
@@ -88,6 +87,7 @@ sound {
 	};
 
 	codec {
+		prefix = "ak4642";
 		sound-dai = <&ak4643>;
 		clocks = <&audio_clock>;
 	};