ASoC: imx: move phycore audmux call into ASoC machine driver

It moves phycore audmux configuration call from board file into ASoC
machine driver phycore-ac97 to ease converting audmux into a platform
driver later.
It moves phycore audmux configuration call from board file into ASoC
machine driver phycore-ac97, so that it gets aligned with wm1133-ev1
and mx27vis-aic32x4, and more importantly it will ease the moving of
audmux into sound/soc/imx as a platform driver later.

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