[CPUFREQ] Fix exposure of ARM_EXYNOS4210_CPUFREQ

exynos4210-cpufreq.c is not buildable on non-exynos builds, so it's
pointless allowing this option to be exposed.  Fix this by adding a
dependency on ARCH_EXYNOS.

drivers/cpufreq/exynos4210-cpufreq.c:20:29: error: mach/regs-clock.h: No such file or directory
drivers/cpufreq/exynos4210-cpufreq.c:21:26: error: mach/cpufreq.h: No such file or directory

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: cpufreq@vger.kernel.org
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Dave Jones <davej@redhat.com>