ASoC: rsnd: add usage for SRC

This patch add missing usage comment for SRC.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87zgy9z9es.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sh/rcar/src.c

@@ -5,6 +5,15 @@
 // Copyright (C) 2013 Renesas Solutions Corp.
 // Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
 
+/*
+ * You can use Synchronous Sampling Rate Convert (if no DVC)
+ *
+ *	amixer set "SRC Out Rate" on
+ *	aplay xxx.wav &
+ *	amixer set "SRC Out Rate" 96000 // convert rate to 96000Hz
+ *	amixer set "SRC Out Rate" 22050 // convert rate to 22050Hz
+ */
+
 /*
  * you can enable below define if you don't need
  * SSI interrupt status debug message when debugging