ARM: EXYNOS4: Add support PPI in external GIC

To support PPI in external GIC of EXYNOS4 SoCs,
gic_arch_extn.irq_eoi, irq_unmask and irq_mask are
fixed. This patch is necessary because external GIC of EXYNOS4
cannot support register banking.

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