ARM: S5PV310: Add irq_mask to handle combiner irqs properly

The 4 combiner groups use same registers to handle the interrupt.
In previous implementation, the whole registers are checked to find
which interupt is occurred and thus interrupt in other groups can
be detected. This patch adds irq_mask to solve this problem.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>