ARM: EXYNOS: Fix build breakage cpuidle on !SMP

The Exynos cpuidle driver has coupled cpuidle built-in so it cannot be
built without SMP:

arch/arm/mach-exynos/pm.c: In function 'exynos_cpu0_enter_aftr':
arch/arm/mach-exynos/pm.c:246:4: error: implicit declaration of function 'arch_send_wakeup_ipi_mask' [-Werror=implicit-function-declaration]
arch/arm/mach-exynos/built-in.o: In function 'exynos_pre_enter_aftr':
../arch/arm/mach-exynos/pm.c:300: undefined reference to 'cpu_boot_reg_base'
arch/arm/mach-exynos/built-in.o: In function 'exynos_cpu1_powerdown':
../arch/arm/mach-exynos/pm.c:282: undefined reference to 'exynos_cpu_power_down'

Fix it by adding missing checks for SMP.

Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>