ARM: S3C24XX: Define a macro to avoid compilation error

Define a macro to avoid the following error during kernel build process
for platforms other than s3c2410:

arch/arm/plat-s3c24xx/cpu.c:84: error: ‘s3c2410a_init’ undeclared here (not in a function)

Signed-off-by: Ramax Lo <ramaxlo@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>