ARM: at91: sama5d3: correct the sound compatible string

As the sama5d3 dtsi file in go into mainline before sound driver,
and, the sound compatible string is changed when go into mainline.
Add this patch to correct the sound compatible string.
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d3xmb.dtsi

@@ -156,7 +156,7 @@ usb2: ehci@00700000 {
 	};
 
 	sound {
-		compatible = "atmel,sama5d3ek-wm8904";
+		compatible = "atmel,asoc-wm8904";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_pck0_as_audio_mck>;