Exynos542x: Add and enable get_periph_rate support

We planned to fetch peripheral rate through one generic API per
peripheral. These generic peripheral functions are in turn
expected to fetch apt values from a function refactored as
per SoC versions. This patch adds support for fetching peripheral
rates for Exynos5420 and Exynos5800.

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>