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: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: NKyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>

arch/arm/mach-exynos/exynos.c

@@ -161,7 +161,9 @@ static void exynos_restart(enum reboot_mode mode, const char *cmd)
 
 static struct platform_device exynos_cpuidle = {
 	.name              = "exynos_cpuidle",
+#ifdef CONFIG_ARM_EXYNOS_CPUIDLE
 	.dev.platform_data = exynos_enter_aftr,
+#endif
 	.id                = -1,
 };