dt-bindings: clock: Add bindings for Exynos850 CMU_APM

CMU_APM generates clocks for APM IP-core (Active Power Management). In
particular it generates RTC clocks, which are needed to enable rtc-s3c
driver on Exynos850 SoC.

Add clock indices and binding documentation for CMU_APM.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20211121232741.6967-2-semen.protsenko@linaro.org