ASoC: fsl-asoc-card: Allow passing the number of slots in use

Currently, fsl-asoc-card supports passing the width of the TDM slot, but
not the number of slots in use, as it harcodes it as two slots.

Add support for passing the number of slots in use.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://lore.kernel.org/r/20230616203913.551183-1-festevam@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>