dts: bindings: boards: Update Ethernet PHY to use `reg` property

Updates Ethernet PHY devicetree bindings to be more consistent with
Linux by using the standard `reg` property for the PHY address instead
of a custom `address` property. As a result, MDIO controller bindings
now require standard `#address-cells` and `#size-cells` properties.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>