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>