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

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 <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1574776274-22355-1-git-send-email-ego@linux.vnet.ibm.com