MAINTAINERS: update MEDIATEK ETHERNET entry

Update MEDIATEK ETHERNET driver maintainer file enty adding myself to
maintainers list
Acked-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -12926,6 +12926,7 @@ M:	Felix Fietkau <nbd@nbd.name>
 M:	John Crispin <john@phrozen.org>
 M:	Sean Wang <sean.wang@mediatek.com>
 M:	Mark Lee <Mark-MC.Lee@mediatek.com>
+M:	Lorenzo Bianconi <lorenzo@kernel.org>
 L:	netdev@vger.kernel.org
 S:	Maintained
 F:	drivers/net/ethernet/mediatek/