MAINTAINERS: add linux-metag mailing list

Add the new linux-metag@vger.kernel.org mailing list to METAG
ARCHITECTURE entry in MAINTAINERS.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org

Author: James Hogan

MAINTAINERS

@@ -5395,6 +5395,7 @@ F:	drivers/watchdog/mena21_wdt.c
 
 METAG ARCHITECTURE
 M:	James Hogan <james.hogan@imgtec.com>
+L:	linux-metag@vger.kernel.org
 S:	Supported
 F:	arch/metag/
 F:	Documentation/metag/