MAINTAINERS: Add the openrisc official repository

The openrisc official repository and patch work happens currently on
github. Add the repo for reference.
Signed-off-by: Stafford Horne <shorne@gmail.com>

MAINTAINERS

@@ -9187,6 +9187,7 @@ OPENRISC ARCHITECTURE
 M:	Jonas Bonn <jonas@southpole.se>
 M:	Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
 M:	Stafford Horne <shorne@gmail.com>
+T:	git git://github.com/openrisc/linux.git
 L:	openrisc@lists.librecores.org
 W:	http://openrisc.io
 S:	Maintained