arc: simplify arc_dma_sync_single_for_{cpu,device}

Remove the indirection through _dma_cache_sync.  Also move the functions
up a bit in the source file as we'll need them in more places soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>