powerpc/powernv: Add documentation for the opal sensor_groups sysfs interfaces

Commit bf9571550f52 ("powerpc/powernv: Add support to clear sensor
groups data") added a mechanism to clear sensor-group data via a sysfs
interface. However, the ABI for that interface has not been
documented.

This patch documents the ABI for the sysfs interface for sensor-groups
and clearing the sensor-groups.

This patch was originally sent by Shilpasri G Bhat on the mailing list:
https://lkml.org/lkml/2018/8/1/85

Signed-off-by: Shilpasri G Bhat <[e-mail]>
Signed-off-by: Gautham R. Shenoy <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-firmware-opal-sensor-groups

···
+What:		/sys/firmware/opal/sensor_groups
+Date:		August 2017
+Contact:	Linux for PowerPC mailing list <[e-mail]>
+Description:	Sensor groups directory for POWER9 powernv servers
+
+		Each folder in this directory contains a sensor group
+		which are classified based on type of the sensor
+		like power, temperature, frequency, current, etc. They
+		can also indicate the group of sensors belonging to
+		different owners like CSM, Profiler, Job-Scheduler
+
+What:		/sys/firmware/opal/sensor_groups/<sensor_group_name>/clear
+Date:		August 2017
+Contact:	Linux for PowerPC mailing list <[e-mail]>
+Description:	Sysfs file to clear the min-max of all the sensors
+		belonging to the group.
+
+		Writing 1 to this file will clear the minimum and
+		maximum values of all the sensors in the group.
+		In POWER9, the min-max of a sensor is the historical minimum
+		and maximum value of the sensor cached by OCC.