sh: Make associative cache writes fatal on all SH-4A parts.

Now that associative cache writes are no longer needed by the SH-4/SH-4A
cache flush code, associative write support can be explicitly disabled
for all SH-4A parts. This makes any associative write throw an exception,
as this behaviour can not be assumed to exist on future parts.

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