cpufreq: exynos: Use static for functions used in only this file

Fixes following sparse error.
drivers/cpufreq/exynos-cpufreq.c:34:5: warning: symbol
'exynos_verify_speed' was not declared. Should it be static?
drivers/cpufreq/exynos-cpufreq.c:40:14: warning: symbol
'exynos_getspeed' was not declared. Should it be static?

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>