ARM: exynos4: convert to CONFIG_MULTI_IRQ_HANDLER

Convert the Exynos4 platforms to be using the gic_handle_irq
function as their primary interrupt handler.

Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>