[Blackfin] arch: add code to initialize globals declared in linux/bootmem.h:...

[Blackfin] arch: add code to initialize globals declared in linux/bootmem.h: max_pfn, max_low_pfn, min_low_pfn.

Signed-off-by: Yi Li <yi.li@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>