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>