ASoC: fsl: Mark 'big-endian' property as optional

Currently the 'big-endian' property is listed as required, which is
not correct. i.MX SoCs do not need such property, so move it under
'Optional properties' entry instead.

Also, fsl-sai.txt incorrectly referenced 'FTM_PWM registers', so
change it to 'SAI registers', which is the intended description.

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