freebsd_network: implement missing generic bus access functions.

Change-Id: I9ed7b0f4187d659df798eac1d01c4f0e1f978e73
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5352
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

freebsd_network: Add generic machine headers.

These can be used on platforms that only support memory spaces
and have no other general restrictions. riscv is one such for now,
so this finally commits support for RISC-V in the FreeBSD compat layer.