target/openrisc: Implement full vmstate serialization

Previously serialization did not persist the tlb, timer, pic and other
key state items.  This meant snapshotting and restoring a running os
would crash. After adding these I am able to take snapshots of a
running linux os and restore at a later time.

I am currently not trying to maintain capatibility with older versions
as I do not believe this really worked before or anyone used it.

Signed-off-by: Stafford Horne <shorne@gmail.com>
