stand/kboot: Make FDT fixup per-arch

The fixups needed vary somewhat by architecture, so move the FDT fixup
to be per-arch. Rename the fdt_linux_fixup() routine to be
fdt_arch_fixup() and expect all architecutres to fix things up as
needed.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36604