Fix min_low_pfn/max_low_pfn build errors for arch/microblaze/: (e.g.)

  ERROR: "min_low_pfn" [drivers/rpmsg/virtio_rpmsg_bus.ko] undefined!
  ERROR: "min_low_pfn" [drivers/hwtracing/intel_th/intel_th_msu_sink.ko] undefined!
  ERROR: "min_low_pfn" [drivers/hwtracing/intel_th/intel_th_msu.ko] undefined!
  ERROR: "min_low_pfn" [drivers/mmc/core/mmc_core.ko] undefined!
  ERROR: "min_low_pfn" [drivers/md/dm-crypt.ko] undefined!
  ERROR: "min_low_pfn" [drivers/net/wireless/ath/ath6kl/ath6kl_sdio.ko] undefined!
  ERROR: "min_low_pfn" [crypto/tcrypt.ko] undefined!
  ERROR: "min_low_pfn" [crypto/asymmetric_keys/asym_tpm.ko] undefined!

Mike had/has an alternate patch for Microblaze:
https://lore.kernel.org/lkml/20200630111519.GA1951986@linux.ibm.com/

David suggested just exporting min_low_pfn & max_low_pfn in
mm/memblock.c:
https://lore.kernel.org/lkml/alpine.DEB.2.22.394.2006291911220.1118534@chino.kir.corp.google.com/Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Michal Simek <monstr@monstr.eu>
Acked-by: David Rientjes <rientjes@google.com>
Cc: linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>