dimm: Implement pseudo device driver for DIMM related drivers

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).