ASoC: pcm512x: Avoid the PLL for the DAC clock, if possible

The PLL introduces jitter, which in turn introduces noice if used
to clock the DAC. Thus, avoid the PLL output, and use the PLL input
to drive the DAC clock, if possible.

This is described for the PCM5142/PCM5242 chips in the answers to the
forum post "PCM5142/PCM5242 DAC clock source" at the TI E2E community
pages (1).

(1) http://e2e.ti.com/support/data_converters/audio_converters/f/64/t/389994



Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>