ASoC: fsl_sai: Add imx6sx platform support

The next coming i.MX6 Solo X SoC also contains SAI module while we use
imp_pcm_init() for i.MX platform.

So this patch adds one compatible route for imx6sx and updates the DT
doc accordingly.

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>