MAINTAINERS: Add stmmac Ethernet driver documentation entry

Add the missing entry for the file that documents the stmicro Ethernet
driver stmmac.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -15777,6 +15777,7 @@ M:	Jose Abreu <joabreu@synopsys.com>
 L:	netdev@vger.kernel.org
 W:	http://www.stlinux.com
 S:	Supported
+F:	Documentation/networking/device_drivers/stmicro/
 F:	drivers/net/ethernet/stmicro/stmmac/
 SUN3/3X