hwmon: (ibmpowernv) Add highest/lowest attributes to sensors

OCC provides historical minimum and maximum value for the sensor
readings. This patch exports them as highest and lowest attributes
for the inband sensors copied by OCC to main memory.

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>