dt-bindings: clock: Document Exynos7885 CMU bindings

Provide dt-schema documentation for Exynos7885 SoC clock controller.
Description is modified from Exynos850 clock controller documentation as
I couldn't describe it any better, that was written by Sam Protsenko.

Signed-off-by: David Virag <virag.david003@gmail.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211206153124.427102-3-virag.david003@gmail.com