ASoC: imx: separate imx-pcm bits from imx-ssi driver

Currently the imx-ssi.c[h] accommodates the imx-pcm common bits which
are shared between imx-pcm-dma-mx2 and imx-pcm-fiq drivers.  It assumes
that imx-pcm-dma-mx2 and imx-pcm-fiq will always be used together with
imx-ssi driver.  However this becomes untrue when we see that driver
sound/soc/fsl/fsl_ssi could possibly work with imx-pcm-dma-mx2 too.

The patch moves the imx-pcm common bits from imx-ssi.c[h] into new
files imx-pcm.c[h], and let imx-pcm-dma-mx2 and imx-pcm-fiq drivers
build it in, so that imx-pcm-dma-mx2 can work with no dependency on
imx-ssi driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>