ARM: dts: imx27-phytec-phycore-som: Use the mux- prefix

According to Documentation/devicetree/bindings/sound/imx-audmux.yaml,
there must be a "mux-" prefix in the audmux port nodes.

Add the "mux-" prefix to avoid the following dt-schema warning:

imx27-phytec-phycore-rdk.dtb: audmux@10016000: 'pins4', 'ssi0' do not match any of the regexes: '^mux-[0-9a-z]*$', 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/sound/imx-audmux.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx27-phytec-phycore-som.dtsi

@@ -49,12 +49,12 @@ &audmux {
 	status = "okay";
 
 	/* SSI0 <=> PINS_4 (MC13783 Audio) */
-	ssi0 {
+	mux-ssi0 {
 		fsl,audmux-port = <0>;
 		fsl,port-config = <0xcb205000>;
 	};
 
-	pins4 {
+	mux-pins4 {
 		fsl,audmux-port = <2>;
 		fsl,port-config = <0x00001000>;
 	};