Blackfin arch: bug fixing restore mach dependent ASYNC memory size

Bug: When SMC921X driver is enabled, kernel boot crash on EZKIT548
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3460



Fixed by restoring mach dependent ASYNC memory size CPLB coverage.
Once we have a more dynamic memory layout we should come up with a better
solution for these hard-coded values.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>