csky: BUGFIX LC-229 dd bs=8192 slow.

When flush_dcache_page is called, we needn't alaways flush the dcache.
Instead, we could set page->flag and flush them in update_mmu_cache().

Change-Id: Iff84cef54a6fd80594247836f74d310d66522527
Signed-off-by: Guo Ren <ren_guo@c-sky.com>