Blackfin: improve async bank access checking (for cross-banks & XIP)

The access_ok() function did not accept ranges within the async banks
which made it impossible to do XIP in flash.  Fixing that also showed
that the current bfin_mem_access_type() code did not work with accesses
that spanned async banks (like a file system).  So split out and fix the
async bank checks so that all these scenarios work as expected.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>