arch/sh: make the DMA mapping operations observe dev->dma_pfn_offset

Some devices may have a non-zero DMA offset, i.e an offset between the
DMA address and the physical address. Such an offset can be encoded
into the dma_pfn_offset field of "struct device", but the SuperH
implementation of the DMA mapping API does not observe this
information.

This commit fixes that by ensuring the DMA address is properly
calculated depending on this DMA offset.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Rich Felker <dalias@libc.org>