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>

Documentation/devicetree/bindings/sound/fsl,ssi.txt

@@ -43,6 +43,10 @@ Required properties:
                     together.  This would still allow different sample sizes,
                     but not different sample rates.
 
+Required are also ac97 link bindings if ac97 is used. See
+Documentation/devicetree/bindings/sound/soc-ac97link.txt for the necessary
+bindings.
+
 Optional properties:
 - codec-handle:     Phandle to a 'codec' node that defines an audio
                     codec connected to this SSI.  This node is typically