[MIPS] Initialize S-cache function pointers even on S-cache-less CPUs.

When a CPU has no scache, the scache flushing functions currently
aren't getting initialized and the NULL pointer is eventually called
as a function.  Initialize the scache flushing functions as a noop
when there's no scache.

Initial patch by me and most of the debugging done by Martin Michlmayr.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>