ASoC: fsl_sai: Make Synchronous and Asynchronous modes exclusive

The previous patch (ASoC: fsl_sai: Add asynchronous mode support) added
new Device Tree bindings for Asynchronous and Synchronous modes support.
However, these two shall not be present at the same time.

So this patch just simply makes them exclusive so as to avoid incorrect
Device Tree binding usage.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>