cpufreq: exynos: Remove unselectable rule for arm-exynos-cpufreq.o

Commit 966f2a71

 ("cpufreq: exynos: remove Exynos4x12 specific
cpufreq driver support") deleted option ARM_EXYNOS_CPUFREQ but missed
to delete a rule in drivers/cpufreq/Makefile which depends on that
option.

Remove unselectable rule for arm-exynos-cpufreq.o
from drivers/cpufreq/Makefile.

Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>