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

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>