cpufreq: s3c24xx: fix "depends on ARM_S3C24XX" in Kconfig

Kconfig symbol S3C24XX_PLL depends on ARM_S3C24XX.  But that symbol
doesn't exist.  Commit f023f8dd ("cpufreq: s3c24xx: move cpufreq
driver to drivers/cpufreq"), which added this issue, makes it clear
that ARM_S3C24XX_CPUFREQ was intended here.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>