Documentation: amd-pstate: Add AMD P-State driver introduction

Introduce the AMD P-State driver design and implementation.

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

Author: huangrui

Documentation/admin-guide/acpi/cppc_sysfs.rst

@@ -4,6 +4,8 @@
 Collaborative Processor Performance Control (CPPC)
 ==================================================
 
+.. _cppc_sysfs:
+
 CPPC
 ====