ASoC: imx: add an explicit Kconfig option for imx-ssi driver

Currently ASoC:imx uses menuconfig option SND_IMX_SOC selects imx-ssi
driver, and it works because all the machine driver covered by the
menuconfig need to build imx-ssi driver in.  However, it will not work
any more if we have a imx based machine driver going into the menuconfig
while working with fsl_ssi driver (sound/soc/fsl/fsl_ssi.c) rather than
imx-ssi one.

The patch adds an explicit Kconfig option SND_SOC_IMX_SSI for imx-ssi
driver, so that it can be selected independently from the menuconfig
option SND_IMX_SOC.

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>