ASoC: imx-ssi: Add .xlate_tdm_slot_mask() support.

This patch add .xlate_tdm_slot_mask support for IMX SSI, and this
will generate the TDM slot TX and RX masks.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>