bhyve: Add FDT building code for arm64

fdt.c provides some basic routines which let platform initialization
code build the FDT that gets passed into the guest. For now this is not
very generic; we declare info about CPUs, memory, a single UART
(specified by -o console), a PCIe controller (used for virtio devices),
an interrupt controller and the platform timer.

Co-authored-by: andrew
Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40996