dt-bindings: clock: Add bindings for Exynos850 CMU_CMGP

CMU_CMGP generates USI and ADC clocks for BLK_ALIVE. In particular USI
clocks are needed for HSI2C_3 and HSI2C_4 instances.

Add clock indices and bindings documentation for CMU_CMGP domain.

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>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20211121232741.6967-4-semen.protsenko@linaro.org