[Blackfin] arch: fix bug - set right partition size in the board files

[Blackfin] arch: fix bug - set right partition size in the board files

 - set default u-boot partition size to 256k
 - modify the offset with the size change
 - use mtd defines (append for offset and full for size)
   where applicable rather than churning constants when we dont have to

Signed-off-by: Grace Pan <grace.pan@analog.com>
Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>