Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Provide wbwd(4), a driver for the watchdog timer found on various
Winbond Super I/O chips.

With minor efforts it should be possible the extend the driver to support
further chips/revisions available from Winbond. In the simplest case
only new IDs need to be added, while different chipsets might require
their own function to enter extended function mode, etc.

Sponsored by: Sandvine Incorporated ULC (in 2011)
Reviewed by: emaste, brueffer
MFC after: 2 weeks