ASoC: audio-graph-card2-custom-sample.dtsi: add convert-rate for DPCM (MIXer)

audio-graph-card2-custom-sample.dtsi is assuming that
DPCM sample is MIXer connection.

FE BE
****
CPU3 -- * * -- Codec3
CPU4 -- * *
****

CPU3/CPU4 need to convert rate in this case.
This patch adds missing "convert-rate" setting sample for it.

Signed-off-by: Kuninori Morimoto <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/generic/audio-graph-card2-custom-sample.dtsi

···
 	 *		+-+		+-+
 	 *
 	 * [DPCM]
+	 *
+	 *	CPU3/CPU4 are converting rate to 44100
+	 *
 	 *	FE		BE
 	 *		  ****
 	 *	cpu3 <-@--*  *--@-> codec3
-	 *	cpu4 <-@--*  *
+	 *	cpu4 <-@--*  *  (44.1kHz)
 	 *		  ****
 	 *
 	 * [DPCM-Multi]
···
 			port@2  { codec2_ep:  endpoint { remote-endpoint = <&mcodec2_ep>; }; };
 
 			/* [DPCM]::BE */
-			port@3  { codec3_ep:  endpoint { remote-endpoint = <&be00_ep>; }; };
+			port@3  {
+				convert-rate = <44100>;
+				codec3_ep:  endpoint { remote-endpoint = <&be00_ep>; };
+			};
 
 			/* [DPCM-Multi]::BE */
 			port@4  { codec4_ep:  endpoint { remote-endpoint = <&mbe1_ep>; }; };