[Blackfin] arch: rename bf5xx-flash to bfin-async-flash

 - move all kconfig board settings into board resources
 - fixup casting style according to lkml feedback
 - rewrite driver so that it can handle arbitrary
   of instances according to the declared platform resources

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Cc: Joern Engel <joern@logfs.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>