[ARM] S3C64XX: add to_irq() support for EINT() GPIO

N group

Add to_irq() function to onvert gpio to irq for external interrupt
group (GPN).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>