ASoC: fsl: rename imx-pcm Kconfig options and filename

Rename a couple of imx-pcm Kconfig options and filename to get them
well named and less confusing.

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