ASoC: fsl: document DT compatible string "fsl,imx-audio-wm8960"

The devicetree compatible string "fsl,imx-audio-wm8960" for
fsl-asoc-card is missing.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>