While gem5 is not qemu, we treat it as "simulators" or "virtual environments".
Add the needed hardcoded gem5 attachments for the UART there, re-using all
the other bits.

In collaboration with: andrew
Sponsored by: DARPA/AFRL
Reviewed by: andrew
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6204