ARM: arm920/s3c24xx: IRQ demulitplexer callback

This patch adds a IRQ demultiplexer callback to the arm920 cpu core code,
plus a stub implementation of it for the S3C2410.

The purpose is to allow arm920t implementations such as the s3c24x0 to
implement interrupt handlers in u-boot without having to touch core
arm920t code.

Signed-off-by: Harald Welte <laforge@openmoko.org>