Blackfin: drop 4KB reserve at end of memory

The point of this small chunk was to avoid anomaly 05000310.  This never
really seemed to do what it was intended though -- no valid CPLBs exist
over the reserved memory, and there is often memory before it anyways (due
to the uClinux MTD and/or reserved DMA region).  Plus, it doesn't address
the L1 instruction case.

So drop this chunk as it wastes memory and is affront to humanity.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>