Blackfin: turn off caches when self initializing

When bootstrapping ourselves on the fly at runtime (via "go"), we need to
turn off the caches to avoid taking software exceptions.  Since caches
need CPLBs and CPLBs need exception handlers, but we're about to rewrite
the code in memory where those exception handlers live, we need to turn
off caches first.

This new code also encourages a slight code optimization by storing the
MMR bases in dedicated registers so we don't have to fully load up the
pointer regs multiple times.

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