ASoC: samsung: i2s: Move saving and restoring regs to runtime pm operations

This patch moves saving and restoring I2S registers to runtime PM
operations, what prepares the driver to operate with audio power domain.
When support for audio power domain is enabled and the domain is being
turned off, the I2S module will loose its context (registers), so runtime
callbacks have to handle it. System sleep suspend/resume operation are
implemented on top of runtime PM operations with generic
pm_runtime_force_suspend/resume helpers.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>