Documentation: Move OpenRISC docs out of arch/

The OpenRISC docs have traditionally been in arch/ but that does not
seem like the correct place to be.  Move them so they will be more
visible to others.  Also update MAINTAINERS to make sure we get
notifications of changes.

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

MAINTAINERS

@@ -10031,6 +10031,7 @@ T:	git git://github.com/openrisc/linux.git
 L:	openrisc@lists.librecores.org
 W:	http://openrisc.io
 S:	Maintained
+F:	Documentation/openrisc/
 F:	arch/openrisc/
 F:	drivers/irqchip/irq-or1k-*