docs: ABI: sysfs-bus-event_source-devices-hv_gpci: Document processor_config sysfs interface file

Add details of the new hv-gpci interface file called
"processor_config" in the ABI documentation.
Reviewed-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230729073455.7918-5-kjain@linux.ibm.com

Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_gpci

@@ -112,3 +112,35 @@ Description:	admin read only
 			   more information.
 
 		* "-EFBIG" : System information exceeds PAGE_SIZE.
+
+What:		/sys/devices/hv_gpci/interface/processor_config
+Date:		July 2023
+Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
+Description:	admin read only
+		This sysfs file exposes the system topology information by making HCALL
+		H_GET_PERF_COUNTER_INFO. The HCALL is made with counter request value
+		PROCESSOR_CONFIG(0x90).
+
+		* This sysfs file will be created only for power10 and above platforms.
+
+		* User needs root privileges to read data from this sysfs file.
+
+		* This sysfs file will be created, only when the HCALL returns "H_SUCCESS",
+		  "H_AUTHORITY" or "H_PARAMETER" as the return type.
+
+		  HCALL with return error type "H_AUTHORITY" can be resolved during
+		  runtime by setting "Enable Performance Information Collection" option.
+
+		* The end user reading this sysfs file must decode the content as per
+		  underlying platform/firmware.
+
+		Possible error codes while reading this sysfs file:
+
+		* "-EPERM" : Partition is not permitted to retrieve performance information,
+			    required to set "Enable Performance Information Collection" option.
+
+		* "-EIO" : Can't retrieve system information because of invalid buffer length/invalid address
+			   or because of some hardware error. Refer to getPerfCountInfo documentation for
+			   more information.
+
+		* "-EFBIG" : System information exceeds PAGE_SIZE.