Documentation: introduce amd-pstate preferrd core mode kernel command line options

amd-pstate driver support enable/disable preferred core.
Default enabled on platforms supporting amd-pstate preferred core.
Disable amd-pstate preferred core with
"amd_prefcore=disable" added to the kernel command line.

Signed-off-by: Meng Li <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Reviewed-by: Wyes Karny <[email protected]>
Reviewed-by: Huang Rui <[email protected]>
Reviewed-by: Perry Yuan <[email protected]>
Tested-by: Oleksandr Natalenko <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Meng Li

Documentation/admin-guide/kernel-parameters.txt

@@ -374,6 +374,11 @@
 			  selects a performance level in this range and appropriate
 			  to the current workload.
 
+	amd_prefcore=
+			[X86]
+			disable
+			  Disable amd-pstate preferred core.
+
 	amijoy.map=	[HW,JOY] Amiga joystick support
 			Map of devices attached to JOY0DAT and JOY1DAT
 			Format: <a>,<b>