ASoC: pcm512x: Use CONFIG_PM_RUNTIME macro

Fixes the following compilation warnings:
sound/soc/codecs/pcm512x.c:520:12: warning: ‘pcm512x_suspend’ defined but not used [-Wunused-function]
sound/soc/codecs/pcm512x.c:545:12: warning: ‘pcm512x_resume’ defined but not used [-Wunused-function]

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>