csky: Fixup defer cache flush for 610

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>