add Stafford as OpenRisc Maintainer, agreed on IRC

MAINTAINERS

@@ -17,5 +17,8 @@ NDS32:
 Vincent Ren-Wei Chen <vincentc@andestech.com>
 Che-Wei Chuang <cnoize@andestech.com>
 
+OpenRisc:
+Stafford Horne <shorne@gmail.com>
+
 Xtensa:
 Max Filippov <jcmvbkbc@gmail.com>