MAINTAINERS: GENET: Add DT binding file

When the DT binding was added in aab5127d ("Documentation: add
Device tree bindings for Broadcom GENET"), the file was not explicitly
listed under the GENET MAINTAINERS section, do that now.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -3580,6 +3580,7 @@ M:	Florian Fainelli <f.fainelli@gmail.com>
 L:	bcm-kernel-feedback-list@broadcom.com
 L:	netdev@vger.kernel.org
 S:	Supported
+F:	Documentation/devicetree/bindings/net/brcm,bcmgenet.txt
 F:	drivers/net/ethernet/broadcom/genet/
 F:	include/linux/platform_data/bcmgenet.h