cpufreq: tegra194: change tegra239_cpufreq_soc to static

tegra239_cpufreq_soc is only used in tegra194-cpufreq.c now, change it
to static.

Fixes: 67688601

 ("cpufreq: tegra194: Add support for Tegra239")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>