ARM: s3c64xx: convert to MULTI_IRQ_HANDLER

Now that there is a generic IRQ handler for multiple VIC devices use it
for s3c64xx to help building multi platform kernels.

Cc: Ben Dooks <ben-linux@fluff.org>
Tested-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>