clk: samsung: Document binding for Exynos4415 clock controller

This patch adds DT binding documentation for Exynos4415 SoC system
clock controllers.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>