ASoC: pcm512x: Add PCM512x driver

The PCM512x devices are a family of monolithic CMOS integrated circuits
that include a stereo digital-to-analog converter and additional support
circuitry.

This is an initial driver which supports some core functionality for the
device which covers common use cases but does not cover all features.
Currently only slave clocking modes with automatic clock configuration
are supported and most of the DSP configuration for the device is not
enabled.

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