[Blackfin] arch: fix bug kernel boot message: memory information is not reasonable

Some of the information in kernel boot message is not reasonable.
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3846

 - use _rambase as the start of kernel image.
   kernel is in the region [_rambase, _ramstart]
 - count in pages in per-cpu-page list as available memory
 - reserved memory now include: [0 - 4K] for bad pointer catching,
   memory reserved for abnormaly 05000263, memory reserved by kernel itself.

Signed-off-by: Yi Li <yi.li@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>