sh: consolidate nommu stubs in arch/sh/mm/nommu.c.

These were previous littered around tlb-nommu.c and pg-nommu.c, though at
this point there are more stubs than are strictly TLB or page op related,
so just consolidate them in a single nommu.c.

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