ASoC: fsl: remove use of imx-pcm-audio from fsl_ssi

Rather than instantiating imx-pcm-audio to call imx_pcm_dma_init(),
fsl_ssi can just directly call it to save the use of imx-pcm-audio.
With this change, fsl_ssi becomes not only a cpu DAI but also a platform
device, so updates platform device setup in imx-sgtl5000 accordingly.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>