openrisc: map as uncached in ioremap

Openrisc is the only architecture not mapping ioremap as uncached,
which has been the default since the Linux 2.6.x days.  Switch it
over to implement uncached semantics by default.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stafford Horne <shorne@gmail.com>