ASoC: fsl-ssi: Add offline_config flag

imx50-ssi and later versions of this IP support online reconfiguration
of all registers. The reference manual does not list any registers that
can only be configured while the SSI unit is disabled. This patch
introduces the flag for later use.

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