clk: exynos: Move pll code into clk-exynos7420

PLL utilities code is only used by clk-exynos7420 driver at the moment.
Move it into clk-exynos7420 to make clk-pll.c file available for CCF PLL
clocks implementation, which is coming in the next patches.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>