ASoC: dt-bindings: fsl-sai: Fix mx6ul and mx7d compatible strings

Describe existing used combinations of compatible strings on MX6UL
and MX7D, which are backward compatible with MX6SX. Add the missing
imx7d-sai compatible string which is used on i.MX7 .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221104160315.213836-1-marex@denx.de
Signed-off-by: Mark Brown <broonie@kernel.org>