ASoC: fsl_sai: Introduce a compatible string for MX6UL

MX6UL may need to configure the General Purpose Register 1 (GPR1), so
it is better to add a new compatible string to differentiate.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>