ARM: S5P: Cleanup S5P gpio interrupt code

This patch performs a global cleanup in s5p gpio interrupt support code.
The code is prepared for upcoming support for gpio interrupts on EXYNOS4
platform, which has 2 gpio banks (regions) instead of one (like on
S5PC110 and S5PC100).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>