dimm: Implement pseudo device driver for DIMM related drivers
- This pseudo device driver installs sensor device for DIMM related
sensors (memtemp(4) and upcoming ecc(4) changes).
- This pseudo device driver keeps DIMM location information. And
the location information is exposed through a new sysctl tree,
hw.dimminfo. The nodes in this sysctl tree have the same names
as the sensor devices, so that programatic way could be used to
report DIMM status.
- This pseudo device implements common sensor update function for
memtemp(4). And it keeps configurable temperature thresholds for
memtemp(4) sensor.
- Utilize this pseudo device driver in memtemp(4).