clk: samsung: exynos-audss: Replace syscore PM with platform device PM

Exynos AUDSS clock driver has been already converted to platform driver,
so remove the dependency on the syscore ops - the last remaining
non-platform driver feature. Platform device's system sleep PM provides
all needed infrastructure for replacing syscore-based PM, so do it now.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>