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