irqchip: exynos: cleanup non-DT stuff in exynos-combiner

For EXYNOS SoCs, only can support for DT so removes non-DT stuff
in exynos-combiner.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>