sparc32: Limit memblock allocation to low memory

Commit cca079ef

 changed sparc32 to use
memblocks instead of bootmem, but also made high memory available via
memblock allocation which does not work together with e.g. phys_to_virt
and can lead to kernel panic.

This changes back to only low memory being allocatable in the early
stages, now using memblock allocation.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>