Add support for the Nuvoton NCT5104D.

Make it compile only for i386/amd64 for now as it's been tested there.
It's quite possible it'll show up elsewhere and we can enable it
for other architectures later.

Tested:

• PC Engines APU1C4

Submitted by: Daniel Wyatt <daniel@dewyatt.com>
Reviewed by: adrian, loos
Differential Revision: https://reviews.freebsd.org/D5389