Blackfin: override default uClinux MTD addr/size

Due to a processor anomaly (05000263 to be exact), most Blackfin parts
cannot keep the embedded filesystem image directly after the kernel in
RAM.  Instead, the filesystem needs to be relocated to the end of memory.
As such, we need to tweak the map addr/size during boot for Blackfin
systems.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>