ASoC: imx: let SND_MXC_SOC_FIQ select FIQ

CONFIG_FIQ is only needed when CONFIG_SND_MXC_SOC_FIQ is selected to
build imx-pcm-fiq.c, so let SND_MXC_SOC_FIQ select FIQ.

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