ASoC: imx: move SND_SOC_AC97_BUS selection down to machine driver

SND_SOC_AC97_BUS is selected to enable the AC97 support in soc-core.
Rather than selecting the option under SND_IMX_SOC, it's better to
leave the selection to individual machine driver which knows if AC97
support is needed or not.

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