ASoC: fsl_ssi: Remove unneeded 'i2s-slave' property

There is no need to use 'i2s-slave' property, since master/slave configuration
are passed via machine layer.

This change does not break existing users because they do check for slave
mode inside sound/soc/fsl/mpc8610_hpcd.c/p1022_ds.c/p1022_rdk.c

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>