sh: Provide the kmap_coherent() interface generically.

This plugs in kmap_coherent() for the non-SH4 cases to permit the
pg-mmu.c bits to be used generically across all CPUs. SH-5 is still in
the TODO state, but will move over to fixmap and the generic interface
gradually.

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