ASoC: bindings: fsl_spdif: Add new compatible string for imx6sx

Add new compatible string "fsl,imx6sx-spdif" in the binding document.
And add compatible string "fsl,vf610-spdif" which was missed before.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/feda3bb02296455d43aeebb7575918d9b28e1a3f.1592376770.git.shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>