Exynos5: Use clock_get_periph_rate generic API

Replacing SoC and peripheral specific function calls with generic
clock_get_periph_rate calls to get the peripheral clocks.
Also, removing dead code of peripheral and SoC specific function
implementations which was used earlier for fetching peripheral clocks.
This code is not being used anymore because of the introduction
of generic clock_get_periph_rate function.

Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>