ARM: EXYNOS: Add irq_domain support for interrupt combiner

Add irq_domain support for hardware interrupts of the interrupt combiner.
The hardware interrupts of all the instances of the combiner are grouped
in a single irq_domain.

Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>