Right now we only have the dma_sync_single_* functions, but no functions
for actually mapping a pointer. The mapping functions become necessary
when casting a pointer to unsigned long to get a dma address is not
enough. (I'm not even going so far that we'll add IOMMU support, but on
some architectures we need a place where we can check if a pointer is
DMA mappable at all)

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>