Blackfin: MPU: handle caches for reserved memory

We weren't handling the user-specified cache behavior for the reserved
memory regions (via mem=/max_mem=).  The no-MPU code already takes care
of this, so add support to the MPU code as well.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>