clk: exynos: Add Samsung clock framework

Heavily based on Linux kernel Samsung clock framework, with some changes
to accommodate the differences in U-Boot CCF implementation. It's also
quite minimal as compared to the Linux version.

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>