ARM: shmobile: koelsch: add sound label on DTS

It is easy to modify settings for Test or other purpose if sound has label

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r8a7791-koelsch.dts

@@ -242,7 +242,7 @@
 			  1800000 0>;
 	};
 
-	sound {
+	rsnd_ak4643: sound {
 		compatible = "simple-audio-card";
 
 		simple-audio-card,format = "left_j";