ASoC: fsl-ssi: Add configuration helper functions

This patch adds a struct 'fsl_ssi_rxtx_reg_val' which holds register
values necessary to enable rx/tx. Based on those preset register values,
the added configuration functions will cleanly enable/disable different
parts of the SSI IP while supporting online/offline configuration.
Different operating modes can be setup directly as different register
values in fsl_ssi_reg_val.

These functions and structs will help to cleanup and simplify the
trigger function to support many different IP versions (online/offline
configuration) and different operating modes.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>