cpufreq: amd-pstate: Add documentation for `amd_pstate_hw_prefcore`

Explain that the sysfs file represents both preferred core being
enabled by the user and supported by the hardware.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Documentation/admin-guide/pm/amd-pstate.rst

@@ -263,6 +263,11 @@ lowest non-linear performance in `AMD CPPC Performance Capability
 <perf_cap_>`_.)
 This attribute is read-only.
 
+``amd_pstate_hw_prefcore``
+
+Whether the platform supports the preferred core feature and it has been
+enabled. This attribute is read-only.
+
 ``energy_performance_available_preferences``
 
 A list of all the supported EPP preferences that could be used for