cpufreq: dt: Add terminate entry for of_device_id tables

Make sure of_device_id tables are NULL terminated.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Fixes: f56aad1d (cpufreq: dt: Add generic platform-device creation support)
CC: 4.7+ <stable@vger.kernel.org> # 4.7+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>