ARM: S3C24XX: add handle_irq function

This removes the dependency on static irq mappings for basic irq handling
and makes the s3c24xx entry-macro.S obsolete.

Also the interrupts of the second full interrupt controller on the s3c2416
are really handled now, which was forgotten when adding them.

The handling itself does the same as the previous assembler-code in that
it tries to get the interrupt offset from the offset register first and
if that produces wrong results manually searches for the interrupt bit
in the pending register value. It also saves the historic comment which
explains the reason behind this.

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