csky/dma: bugfix dma_sync_for_cpu/device

ref: https://lkml.org/lkml/2018/5/18/1068



        map             for_cpu         for_device      unmap
TO_DEV  writeback       none            writeback       none
TO_CPU  invalidate      invalidate*     invalidate      invalidate*
BIDIR   writeback       invalidate      writeback       invalidate

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