sh: rename pg-mmu.c -> cache.c, enable generically.

This builds in the newly created cache.c (renamed from pg-mmu.c) for both
MMU and NOMMU configurations. The kmap_coherent() stubs and alias
information recorded by each CPU family takes care of doing the right
thing while enabling the code to be commonly shared.

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