dt-bindings: net: brcm,unimac-mdio: Update maintainers for binding

Add Doug and myself as maintainers since this binding is used by the
GENET Ethernet controller for its internal MDIO controller.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211206180049.2086907-3-f.fainelli@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>