MIPS: Synchronize dma_map_page and dma_map_single

Synchronize dma_map_page/dma_unmap_page and dma_map_single/dma_unmap_single.
This will reduce unnecessary writebacks and invalidates.

[Ralf: make dma_unmap_page an inline function.]

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>