cpufreq: Fix sparse warnings by updating cputime64_t to u64

There were few sparse warnings due to mismatch of type on function arguments.
Two types were used u64 and cputime64_t. Both are actually u64, so use u64 only.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>