ARM: S3C24XX: fix irq parent check

The current parent_irq check checks for a value != 0. This does of course
not work when the parent irq sits in the bit 0 of the parent register.
This only affects the eint0 interrupt of the s3c2412.

To fix this behaviour, check for the presence of a parent_intc in the
structure. In an s3c24xx interrupt controller either all interrupts have
parent interrupts or none have, so if a parent controller is available
the parent_irq value always points to a parent_irq.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>