Exynos542x: Move exynos5420_get_pll_clk up and rename

Exynos542x: Move exynos5420_get_pll_clk up and rename

Moving exynos5420_get_pll_clk function definition up in the
code to keep it together with rest of SoC_get_pll_clk functions.
This makes code more legible and also removes the need of
declaration when called before the position of definition in
code. Also, renaming exynos5420_get_pll_clk to
exynos542x_get_pll_clk because it is being used for both Exynos
5420 and 5800.

Signed-off-by: Akshay Saraswat <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Tested-by: Simon Glass <[email protected]>
Signed-off-by: Minkyu Kang <[email protected]>