Audio : add sound-ext-card to support more sound-card.

Change-Id: I8d010d92c8fe3a7c42f01691c8987bed8b106dd2

Author: yupei_lin

arch/arm/boot/dts/rk3288-tinker-board.dtsi

@@ -101,6 +101,10 @@
 	};
 */
 
+	sound-ext-card {
+		status = "disabled";
+	};
+
 	ext_gmac: external-gmac-clock {
 		compatible = "fixed-clock";
 		clock-frequency = <125000000>;