85xx: Improve flash remapping on MPC8572DS & MPC8536DS

Changing the flash from cacheable to cache-inhibited was taking a significant
amount of time due to the fact that we were iterating over the full 256M of
flash.  Instead we can just flush the L1 d-cache and invalidate the i-cache.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>