ARM: EXYNOS: Add cpu-offset property in gic device tree node

Commit db0d4db2 ('ARM: gic: allow GIC to support non-banked setups)
requires a cpu-offset property to be specified for non-banked gic
controllers, which is the case for Exynos4.

Reported-and-Tested-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>