smbios: handle smbios3 for arm64

Get smbios working on arm64 where it seems to be
exclusively smbios version 3.x

The "interesting" thing here is that the smbios table seems to be
RAM in the EFI runtime services table. This makes it owned by "ram0",
and not io memory. That prevents bus_alloc_resource() from being able
to claim it, since ram0 already owns it. According to jhb, this is
how things are supposed to work. Eg, bus_alloc_resource() is meant
to be used with IO memory, not physical memory. Following his
suggestion, I converted the driver to simply use pmap_mapbios().

This is a prerequisite for getting IPMI to attach via the SSIF
attachment on arm64 servers, where all IPMI that I've seen
uses SSIF.

Note that this change is based on initial work by Allan Jude in
https://reviews.freebsd.org/D28739.

Reviewed by: imp
Sponsored by: Netflix, Ampere Computing LLC (D28739)
Differential Revision: https://reviews.freebsd.org/D42592