MN10300: Make the boot wrapper able to use writeback caching

Make the boot wrapper able to use writeback caching, including flushing the
cache before jumping to the main kernel.

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>