clk: exynos: Add Exynos850 clock driver

Heavily influenced by its Linux kernel counterpart. It's implemented on
top of recently added Samsung CCF clock framework API. For now only UART
leaf clocks are implemented, along with all preceding clocks in CMU_TOP
and CMU_PERI. The UART baud clock is required in the serial driver, to
get its rate for the consequent baud rate calculation.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>