cpufreq: amd-pstate: Document the units for freq variables in amd_cpudata

cpufreq: amd-pstate: Document the units for freq variables in amd_cpudata

The min_limit_freq, max_limit_freq, min_freq, max_freq, nominal_freq
and the lowest_nominal_freq members of struct cpudata store the
frequency value in khz to be consistent with the cpufreq
core. Update the comment to document this.

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>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>