cpufreq: Make CONFIG_CPUFREQ_DT_PLATDEV depend on OF

The cpufreq-dt-platform.c driver requires CONFIG_OF to be selected. Mark
it as a dependency.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306250025.savpMM8L-lkp@intel.com/


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