clk: samsung: exynos5250: Move sleep init function to init section

The exynos5250_clk_sleep_init() function can be moved to init section
because it is referenced only from other init-level calls.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>