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

sound/soc/imx/Kconfig

 menuconfig SND_IMX_SOC
 	tristate "SoC Audio for Freescale i.MX CPUs"
 	depends on ARCH_MXC
-	select SND_SOC_AC97_BUS
 	help
 	  Say Y or M if you want to add support for codecs attached to
 	  the i.MX SSI interface.
@@ -38,6 +37,7 @@ config SND_SOC_MX27VIS_AIC32X4
 config SND_SOC_PHYCORE_AC97
 	tristate "SoC Audio support for Phytec phyCORE (and phyCARD) boards"
 	depends on MACH_PCM043 || MACH_PCA100
+	select SND_SOC_AC97_BUS
 	select SND_SOC_WM9712
 	select SND_MXC_SOC_FIQ
 	help