clk: samsung: exynos7: add clocks for RTC block

Add clock support for the RTC block in Exynos7.

Signed-off-by: Naveen Krishna Ch <naveenkrishna.ch@gmail.com>
Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>