Basic OPAL sensor support for POWER9 platforms

Summary:
PowerNV architectures (in the test case POWER9) export sensors via the device
tree, which are accessed via OPAL calls. This adds sysctl nodes for each
device in a generic fashion. New sysctl nodes are:

dev.opal_sensor.N.sensor
dev.opal_sensor.N.sensor_min
dev.opal_sensor.N.sensor_max
dev.opal_sensor.N.type
dev.opal_sensor.N.label

These are rooted at a parent attachment under opal, called opalsens. This does
not add support for the "sensor groups" defined in the device tree.

Reviewed by: breno.leitao_gmail.com
Differential Revision: https://reviews.freebsd.org/D15362