cpufreq: powernv: Make some symbols static

The sparse tool complains as follows:

drivers/cpufreq/powernv-cpufreq.c:88:1: warning:
 symbol 'pstate_revmap' was not declared. Should it be static?
drivers/cpufreq/powernv-cpufreq.c:383:18: warning:
 symbol 'cpufreq_freq_attr_cpuinfo_nominal_freq' was not declared. Should it be static?
drivers/cpufreq/powernv-cpufreq.c:669:6: warning:
 symbol 'gpstate_timer_handler' was not declared. Should it be static?
drivers/cpufreq/powernv-cpufreq.c:902:6: warning:
 symbol 'powernv_cpufreq_work_fn' was not declared. Should it be static?

Those symbols are not used outside of this file, so mark
them static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>