sh: Fix dcache flushing for N-way write-through caches.

This adopts the special-cased 2-way write-through dcache flusher for
N-ways and moves it in to the generic path. Assignment is done at runtime
via the check for the CCR_CACHE_WT bit in the same path as the per-way
writeback flushers.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>