cpufreq: docs: Update core.rst

As the definition of struct cpufreq_freqs has changed, update core.rst
with the new first member of struct cpufreq_freqs.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>