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

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 febab20caeba("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: febab20caeba("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>