MN10300: AM34: Add cacheflushing by using the AM34 purge registers

The AM34 CPU core provides an automated way of purging the cache rather than
manually iterating over all the tags in the cache.  Make it possible to use
these.

Signed-off-by: Akira Takeuchi <takeuchi.akr@jp.panasonic.com>
Signed-off-by: Kiyoshi Owada <owada.kiyoshi@jp.panasonic.com>
Signed-off-by: David Howells <dhowells@redhat.com>