ASoC: fsl_spdif: Add sysclk df support to derive txclk from sysclk

The sysclk is one the clock sources that could be selected to derive
tx clock. But the route for sysclk is a bit different that it does
not only contain txclk df divider but also have an extra sysclk df.

So this patch mainly adds syclk df configuration support so as to
let the driver be able to get clock from sysclk.

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