dt-bindings: clock: Add Exynos850 clock controller

Add bindings documentation and the header file for Exynos850 clock
controller. It was taken from Linux kernel [1,2].

[1] Documentation/devicetree/bindings/clock/samsung,exynos850-clock.yaml
[2] include/dt-bindings/clock/exynos850.h

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