[coreboot] coreboot specific ACPI table

[Julius Werner]

I think we should only export the coreboot table location and size
through ACPI and then read everything else from there. That way we can
share almost all of the kernel driver code with ARM and just need a
tiny platform-specific stub to look up the table location first. If we
add all the information into an actual ACPI table, we're building an
x86-only solution again. (A generic, platform-independent coreboot
driver could just as easily export the stuff we want into sysfs.)

For comparison, the coreboot device tree interface on ARM
(https://lkml.org/lkml/2014/6/16/622, unfortunately never picked up by
the maintainers but still used on Chromebooks today) only exports
start address and size of the coreboot table and the whole CBMEM area,
so maybe ACPI should just match that. (I guess if CBMEM is referenced
by the coreboot table like Aaron said, we could also drop that part.)