ARM: dts: imx6: 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 devicetree schema warnings.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>