dt-bindings: net: mdio: Add address and size cells

MDIO controllers should have child nodes with a singe value in their reg
properties being the ID of the PHY in the bus.

Let's add the proper constraints on #address-cells and #size-cells.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>