dt-bindings: dsa: Remove unnecessary #address/#size-cells

If the example binding is used on a real dts file, the following DTC
warning is seen with W=1:

arch/arm/boot/dts/imx6q-b450v3.dtb: Warning (avoid_unnecessary_addr_size): /mdio-gpio/switch@0: unnecessary #address-cells/#size-cells without "ranges" or child "reg" property

Remove unnecessary #address-cells/#size-cells to improve the binding
document examples.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>