cpufreq: pcc-cpufreq: Mark sometimes used ID structs as __maybe_unused

Not used when MODULE is not defined.

Fixes the following W=1 kernel build warning(s):

 drivers/cpufreq/pcc-cpufreq.c:619:36: warning: ‘processor_device_ids’ defined but not used [-Wunused-const-variable=]
 619 | static const struct acpi_device_id processor_device_ids[] = {
 | ^~~~~~~~~~~~~~~~~~~~

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