ARM: SAMSUNG: Fix build error when selecting CPU_FREQ_S3C24XX_DEBUGFS on S3C2440

Following is happened when CONFIG_CPU_FREQ_S3C24XX_DEBUGFS
is selected without building of s3c2410-iotiming.c file:

arch/arm/mach-s3c2440/built-in.o:(.data+0x38c): undefined reference to `s3c2410_iotiming_debugfs

Basically, the CONFIG_S3C2410_IOTIMING is not selected for
MACH_MINI2440. Because the s3c2410-iotiming.c is not ever
compiled and enabling CONFIG_CPU_FREQ_S3C24XX_DEBUGFS option
caused undefined reference to s3c2410_iotiming_debugfs()
defined in that file.  The s3c2410_iotiming_debugfs defined
as NULL for this case.

Signed-off-by: Denis Kuzmenko <linux@solonet.org.ua>
Cc: stable@kernel.org
[kgene.kim@samsung.com: removed useless changes]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>