ARM: EXYNOS: Fix incorrect initialization of GIC

Use the of_irq_init() call to setup the gic which also properly
registers the gic device node pointer with gic irq domain,
without which all interrupt specifier translations for gic fail.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>