MAINTAINERS: GENET: Add UniMAC MDIO controller files

In preparation for removing myself from the PHYLIB entry, add the UniMAC
MDIO controller files (DT binding, driver and platform_data header) to
the GENET entry. The UniMAC MDIO controller is essential to the GENET
operation, therefore it makes sense to group them together.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>