clk: samsung: exynos-audss: Add support for runtime PM

This patch adds support for runtime PM to Exynos Audio SubSystem driver to
enable full support for audio power domain on Exynos5 SoCs. The main change
is moving register saving and restoring code from system sleep PM ops to
runtime PM ops and implementing system sleep PM ops with generic
pm_runtime_force_suspend/resume helpers. Runtime PM of the Exynos AudSS
device is managed from clock core depending on the preparation status
of the provided clocks.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1503302703-13801-6-git-send-email-m.szyprowski@samsung.com