MAINTAINERS: add etnaviv mailinglist

The etnaviv project now has its own mailinglist, add it to the
MAINTAINERS file, so kernel patches get CC'ed to it.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

MAINTAINERS

@@ -4242,6 +4242,7 @@ DRM DRIVERS FOR VIVANTE GPU IP
 M:	Lucas Stach <l.stach@pengutronix.de>
 R:	Russell King <linux+etnaviv@armlinux.org.uk>
 R:	Christian Gmeiner <christian.gmeiner@gmail.com>
+L:	etnaviv@lists.freedesktop.org
 L:	dri-devel@lists.freedesktop.org
 S:	Maintained
 F:	drivers/gpu/drm/etnaviv/