We use defer cache flush mechanism to improve the performance of
610, but the implementation is wrong. We fix it up now and update
the mechanism:

 - Zero page needn't be flushed.
 - If page is file mapping & non-touched in user space, defer flush.
 - If page is anon mapping or dirty file mapping, flush immediately.
 - In update_mmu_cache finish the defer flush by flush_dcache_page().

For 610 we need take care the dcache aliasing issue:
 - VIPT cache with 8K-bytes size per way in 4K page granularity.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>