ARM: dts: imx51-babbage: Pass a real clock to the codec

On imx51_babbage the codec clock is activated via GPIO4_26.

Provide a real clock to the sgtl5000 codec via device tree.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>