Blackfin: decouple unrelated cache settings to get exact behavior

The current cache options don't really represent the hardware features.
They end up setting different aspects of the hardware so that the end
result is to turn on/off the cache.  Unfortunately, when we hit cache
problems with the hardware, it's difficult to test different settings to
root cause the problem.  The current settings also don't cleanly allow for
different caching behaviors with different regions of memory.

So split the configure options such that they properly reflect the settings
that are applied to the hardware.

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