ARM: EXYNOS: Remove build warning without enabling PM

Fixed following build warning with exynos4_defconfig.
arch/arm/mach-exynos/clock.c:33: warning: 'exynos4_clock_save' defined but not used
arch/arm/mach-exynos/clock-exynos4210.c:35: warning: 'exynos4210_clock_save' defined but not used
arch/arm/mach-exynos/clock-exynos4212.c:35: warning: 'exynos4212_clock_save' defined but not used

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>