sh: bootmem refactoring.

This reworks much of the bootmem setup and initialization code allowing
us to get rid of duplicate work between the NUMA and non-NUMA cases. The
end result is that we end up with a much more flexible interface for
supporting more complex topologies (fake NUMA, highmem, etc, etc.) which
is entirely LMB backed. This is an incremental step for more NUMA work as
well as gradually enabling migration off of bootmem entirely.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>