cpufreq: amd-pstate: Document *_limit_* fields in struct amd_cpudata

The four fields of struct cpudata namely min_limit_perf,
max_limit_perf, min_limit_freq, max_limit_freq introduced in the
commit febab20c("cpufreq/amd-pstate: Fix scaling_min_freq and
scaling_max_freq update") are currently undocumented

Add comments describing these fields
Acked-by: Huang Rui <ray.huang@amd.com>
Fixes: febab20c("cpufreq/amd-pstate: Fix scaling_min_freq and scaling_max_freq update")
Reviewed-by: Li Meng <li.meng@amd.com>
Tested-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>
Signed-off-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/amd-pstate.h

@@ -49,6 +49,10 @@ struct amd_aperf_mperf {
  * @lowest_perf: the absolute lowest performance level of the processor
  * @prefcore_ranking: the preferred core ranking, the higher value indicates a higher
  * 		  priority.
+ * @min_limit_perf: Cached value of the performance corresponding to policy->min
+ * @max_limit_perf: Cached value of the performance corresponding to policy->max
+ * @min_limit_freq: Cached value of policy->min
+ * @max_limit_freq: Cached value of policy->max
  * @max_freq: the frequency that mapped to highest_perf
  * @min_freq: the frequency that mapped to lowest_perf
  * @nominal_freq: the frequency that mapped to nominal_perf