ASoC: imx/mxs: remove redundant SND_PCM selection

SND_PCM is already selected by SND_SOC, there is no need for
SND_IMX_SOC and SND_MXS_SOC to select it again.

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