irqchip: s3c24xx: add devicetree support

Add the necessary code to initialize the interrupt controller
thru devicetree data using the irqchip infrastructure.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>