jedec_ts: a driver for thermal sensors on memory modules

The driver currently supports chips that are fully compliant with the
JEDEC SPD / EEPROM / TS standard (JEDEC Standard 21-C,
TSE2002 Specification, frequenlty referred to as JEDEC JC 42.4).

Additionally some chips from STMicroelectronics are supported as well.
They are compliant except for their Device ID pattern.

Given the continued lack of any common sensor infrastructure, the driver
uses an ad-hoc sysctl to report the temperature.

Reviewed by: wblock (documentation)
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D8174