ASoC: fsl-ssi: ac97-slave support

This patch adds ac97-slave support.

For ac97, the registers have to be setup earlier than for other ssi
modes because there is some communication with the external device
before streaming. So this patch introduces a fsl_ssi_setup function to
setup the registers for different ssi operation modes seperately.

This patch was tested with imx27-pca100.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>