clk: samsung: Add local variable to match its purpose

Add new variable to avoid using clk pointer for different purposes across
the exynos_register_cpu_clock() function. This will help in future rewrite
for the new clk_hw API.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>