EXYNOS: fix dependency for EXYNOS_CPUFREQ

This fixes the CPUFREQ dependency for regarding EXYNOS SoCs
such as EXYNOS4210, EXYNOS4X12 and EXYNOS5250. Its cpufreq
driver should be built with selection of SoC arch part.

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