Documentation/ABI: Add ABI documentation for hv-gpci format

This patch adds ABI documentation for hv-gpci event format.
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201003074943.338618-3-kjain@linux.ibm.com

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

+What:           /sys/bus/event_source/devices/hv_gpci/format
+Date:           September 2020
+Contact:        Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
+Description:    Read-only. Attribute group to describe the magic bits
+                that go into perf_event_attr.config for a particular pmu.
+                (See ABI/testing/sysfs-bus-event_source-devices-format).
+
+                Each attribute under this group defines a bit range of the
+                perf_event_attr.config. All supported attributes are listed
+                below.
+
+				counter_info_version  = "config:16-23"
+				length  = "config:24-31"
+				partition_id  = "config:32-63"
+				request = "config:0-31"
+				sibling_part_id = "config:32-63"
+				hw_chip_id = "config:32-63"
+				offset = "config:32-63"
+				phys_processor_idx = "config:32-63"
+				secondary_index = "config:0-15"
+				starting_index = "config:32-63"
+
+               For example,
+
+		processor_core_utilization_instructions_completed = "request=0x94,
+					phys_processor_idx=?,counter_info_version=0x8,
+					length=8,offset=0x18"
+
+		In this event, '?' after phys_processor_idx specifies this value
+		this value will be provided by user while running this event.
+
 What:		/sys/bus/event_source/devices/hv_gpci/interface/collect_privileged
 Date:		February 2014
 Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>