ramips: rt3883: fix dtc compiler warning

Fixes the following compiler warning:

Warning (reg_format): "reg" property in /ethernet@10100000/port@0 has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
Warning (avoid_default_addr_size): Relying on default #address-cells value for /ethernet@10100000/port@0
Warning (avoid_default_addr_size): Relying on default #size-cells value for /ethernet@10100000/port@0

Signed-off-by: Mathias Kresin <dev@kresin.me>