ARM: EXYNOS4: Add support external GIC

For full support of power modes, this patch adds implementation
external GIC on EXYNOS4.

External GIC of Exynos4 cannot support register banking so
several interrupt related code for CPU1 should be different
from that of CPU0.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>