ARM: EXYNOS4: Support early wakeup entering sleep mode

Since early wakeup can be handled in pm so we don't need masking
interrupts of external GIC. When the early wakeup interrupt happens,
PMU(Power Management Unit) ignores WFI instruction. This means that
PC(Program Counter) passed without any changes. This patch can handle
that case by early wakeup interrupt.

Signed-off-by: Jaecheol Lee <jc.lee@samsung.com>
[kgene.kim@samsung.com: fixed return of exynos4_cpu_suspend()]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>