bhyve: Move the RTC driver to amd64/

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40734

(cherry picked from commit a7f6c2ff732db1d0032066109ee0dd5af1a39b81)