clk: exynos: Add header guard for clk-pll.h

The clk-pll.h is going to be included in multiple files soon. Add
missing header guard to prevent possible build errors in future.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: 166097e8 ("clk: exynos: add clock driver for Exynos7420 Soc")
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>