irqchip: s3c24xx: globally keep track of the created intc instances

For dt-enabled machines we want to use a big irq_domain over all controllers
and therefore need to access not only the main controllers but the
sub-controller as well.

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