sh: Bump up dma_ops initialization far earlier in the boot process.

Presently this was tacked on to the dma debug init bits from
fs_initcall(), which is far too late for devices setting up their own
per-device coherent areas.

Throw this in the beginning of mem_init(), as per the x86 iommu
allocation.

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