nds32: To simplify the implementation of update_mmu_cache()

The checking code is done in kmap_atomic() so that we don't need to
check it in update_mmu_cache() again. There is no need to implement
it for cache aliasing or cache non-aliasing versions. We can just
implement one version for both.

Signed-off-by: Greentime Hu <greentime@andestech.com>