clk: exynos-audss: convert to platform device

The Exynos AudioSS clock controller will later be modified to allow
input clocks to be specified via device-tree in order to support
multiple Exynos SoCs.  This will introduce a dependency on the core
SoC clock controller being initialized first so that the AudioSS driver
can look up its input clocks, but the order in which clock providers
are probed in of_clk_init() is not guaranteed.  Since deferred probing
is not supported in of_clk_init() and the AudioSS block is not the core
controller, we can initialize it later as a platform device.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>