ARM: S5P64X0: External Interrupt Support

Add external interrupt support for S5P64X0.The external interrupt
group 0(0 to 15) is used for wake-up source in stop and sleep mode.
Add generic irq chip support

Signed-off-by: Padmavathi Venna <padma.v@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>