microblaze: fix min_low_pfn/max_low_pfn build errors

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/[email protected]/

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 <[email protected]>
Signed-off-by: Randy Dunlap <[email protected]>
Acked-by: Michal Simek <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: [email protected]
Cc: Andrew Morton <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: Michal Simek <[email protected]>
Signed-off-by: Mike Rapoport <[email protected]>

arch/microblaze/mm/init.c

@@ -46,6 +46,9 @@ unsigned long memory_size;
 EXPORT_SYMBOL(memory_size);
 unsigned long lowmem_size;
 
+EXPORT_SYMBOL(min_low_pfn);
+EXPORT_SYMBOL(max_low_pfn);
+
 #ifdef CONFIG_HIGHMEM
 pte_t *kmap_pte;
 EXPORT_SYMBOL(kmap_pte);