ARM: S3C2410: Fix link if CONFIG_S3C2410_IOTIMING is not set

Fix the link errors if cpu-frequency support is enabled on s3c2410 systems
but there is no CONFIG_S3C2410_IOTIMING set. Fix this by ensuring the
relevant symbols are defined NULL if the code is not being built in.

Fixes the following error:
arch/arm/mach-s3c2410/built-in.o: undefined reference to `s3c2410_iotiming_get'
arch/arm/mach-s3c2410/built-in.o: undefined reference to `s3c2410_iotiming_set'
arch/arm/mach-s3c2410/built-in.o: undefined reference to `s3c2410_iotiming_calc'

Signed-off-by: Ben Dooks <ben-linux@fluff.org>