sh64: Move consistent DMA routines to arch/sh64/mm/.

The consistent DMA stuff was evolved from the old PCI DMA mapping
functions. As this has been overhauled, it's something that is
used by the generic DMA mapping code, and thus, has no intrinsic
PCI dependence.

Move the routines somewhere more sensible (same place as sh), and
fix up the build for CONFIG_PCI=n in the process.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>