sensors-detect: Add detection of NCT6791D

New superio chip, found on recent ASUS boards.
sensors-detect: Add detection of NCT6681D, NCT6682D, and NCT6683D

NCT6683D is found on recent Intel mainboards (eg DH87MC, DH87RL).
NCT6681D is mentioned on the Nuvoton web site. NCT6682D is similar
to NCT6681D with added support for current monitoring.