ARM: EXYNOS: Fix build with PM_SLEEP=n and ARM_EXYNOS_CPUIDLE=n

Fix building of exynos_defconfig with disabled CONFIG_PM_SLEEP by
adding checking whether Exynos cpuidle support is enabled before
accessing exynos_enter_aftr.

The build error message:
arch/arm/mach-exynos/built-in.o:(.data+0x74): undefined reference to `exynos_enter_aftr'
make: *** [vmlinux] Error 1

This patch has been tested on Exynos4210 based Origen board.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>