wbwd: move to superio(4) bus

This allows to remove a bunch of low level code.
Also, superio(4) provides safer interaction with other drivers
that work with Super I/O configuration registers.

Tested only on PCengines APU2:
superio0: <Nuvoton NCT5104D/NCT6102D/NCT6106D (rev. B+)> at port 0x2e-0x2f on isa0
wbwd0: <Nuvoton NCT6102 (0xc4/0x53) Watchdog Timer> at WDT ldn 0x08 on superio0

The watchdog output is incorrectly wired on that system and the watchdog
does not really do it its job, but the pulse can be seen with a signal
analyzer.

Reviewed by: delphij, bcr (man page)
MFC after: 19 days
Differential Revision: https://reviews.freebsd.org/D21979