hwmon: Add power meter spec to Documentation/hwmon/sysfs-interface

Update the hwmon sysfs interface documentation to include a specification
for power meters.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>