ARM: dts: imx51-zii-rdu1: Fix audmux node names

Rename the 'ssi2' and 'aud3' nodes to 'mux-ssi2' and 'mux-aud3' in the
audmux configuration of imx51-zii-rdu1.dts to comply with the naming
convention in imx-audmux.yaml.

This fixes the following dt-schema warning:

  imx51-zii-rdu1.dtb: audmux@83fd0000 (fsl,imx51-audmux): 'aud3', 'ssi2'
  do not match any of the regexes: '^mux-[0-9a-z]*$', '^pinctrl-[0-9]+$'

Fixes: ceef039 ("ARM: dts: imx: add ZII RDU1 board")
Signed-off-by: Jihed Chaibi <jihed.chaibi.dev@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: JihedChaibi

arch/arm/boot/dts/nxp/imx/imx51-zii-rdu1.dts

@@ -259,7 +259,7 @@
 	pinctrl-0 = <&pinctrl_audmux>;
 	status = "okay";
 
-	ssi2 {
+	mux-ssi2 {
 		fsl,audmux-port = <1>;
 		fsl,port-config = <
 			(IMX_AUDMUX_V2_PTCR_SYN |
@@ -271,7 +271,7 @@
 		>;
 	};
 
-	aud3 {
+	mux-aud3 {
 		fsl,audmux-port = <2>;
 		fsl,port-config = <
 			IMX_AUDMUX_V2_PTCR_SYN