hwmon: (ibmpowernv) add a label attribute

Currently, sensors are only identified by their type and index.

The new OPAL device tree can expose extra properties to identify
some sensors by their name or location. This patch adds the creation
of a new hwmon *_label attribute when such properties are detected.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>