Documentation: amd-pstate: introduce new global sysfs attributes

The amd-pstate driver supports switching working modes at runtime.
Users can view and change modes by interacting with the "status" sysfs
attribute.

1) check driver mode:
$ cat /sys/devices/system/cpu/amd-pstate/status

2) switch mode:
`# echo "passive" | sudo tee /sys/devices/system/cpu/amd-pstate/status`
or
`# echo "active" | sudo tee /sys/devices/system/cpu/amd-pstate/status`

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Wyes Karny <wyes.karny@amd.com>
Tested-by: Wyes Karny <wyes.karny@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Perry Yuan

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

@@ -339,6 +339,35 @@ processor must provide at least nominal performance requested and go higher if c
 operating conditions allow.
 
 
+User Space Interface in ``sysfs``
+=================================
+
+Global Attributes
+-----------------
+
+``amd-pstate`` exposes several global attributes (files) in ``sysfs`` to
+control its functionality at the system level.  They are located in the
+``/sys/devices/system/cpu/amd-pstate/`` directory and affect all CPUs.
+
+``status``
+	Operation mode of the driver: "active", "passive" or "disable".
+
+	"active"
+		The driver is functional and in the ``active mode``
+
+	"passive"
+		The driver is functional and in the ``passive mode``
+
+	"disable"
+		The driver is unregistered and not functional now.
+
+        This attribute can be written to in order to change the driver's
+        operation mode or to unregister it.  The string written to it must be
+        one of the possible values of it and, if successful, writing one of
+        these values to the sysfs file will cause the driver to switch over
+        to the operation mode represented by that string - or to be
+        unregistered in the "disable" case.
+
 ``cpupower`` tool support for ``amd-pstate``
 ===============================================