ASoC: fsl: make fsl_ssi driver compilable on ARM/IMX

Provide different pair of accessors for accessing SSI registers on
PowerPC and ARM/IMX, so that fsl_ssi driver can be built on both
architectures.

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