ARM: S3C24XX: remove plat/irq.h in plat-samsung

plat-samsung/irq.h did only contain functions for handling the spread out
subirqs on s3c24xx arches, which are not needed anymore.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[kgene.kim@samsung.com: fixed build error on bast-irq.c]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>