arm: dma-mapping.c: add cpu sync in map_sg and unmap_sg

Currently for 32-bit the dma operations including map_sg
and unmap_sg don't have cpu sync, so after several times'
map and unmap, the data may be not right.

This is caused by 'commit 14072e39 ("arm: dma-mapping:
map sg lists into the SMMU as virtually contiguous")'.

Change-Id: I6b749f5111aa937f7800050ae454fe209b73c5d1
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>