MAINTAINERS: GENET: Add missing platform data file

When commit b0ba512e ("net: bcmgenet: enable driver to work without
a device tree") added include/linux/platform_data/bcmgenet.h, the file
was not added to the GENET MAINTAINERS file section, add it now.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -3580,6 +3580,7 @@ L:	bcm-kernel-feedback-list@broadcom.com
 L:	netdev@vger.kernel.org
 S:	Supported
 F:	drivers/net/ethernet/broadcom/genet/
+F:	include/linux/platform_data/bcmgenet.h
 
 BROADCOM IPROC ARM ARCHITECTURE
 M:	Ray Jui <rjui@broadcom.com>