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.

Most imported from https://github.com/zxombie/freebsd/tree/bhyvearm64 ,
with some refactoring to avoid hard-coding various constants which
define the location of MMIO regions.