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

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: ceef0396f367f ("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>