ASoC: fsl: separate SSI and DMA Kconfig options

The fsl_ssi driver will possibly be shared between Freescale PowerPC
and ARM/IMX families, so give it a separate Kconfig option.  Then
fsl_ssi driver can possibly be selected independently from selecting
PowerPC DMA based PCM driver.

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>