ARM: dts: BCM5301X: Fix MDIO node address/size cells

The MDIO node on BCM5301X had an reversed #address-cells and
 #size-cells properties, correct those, silencing checker warnings:

.../linux/arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dt.yaml: mdio@18003000: #address-cells:0:0: 1 was expected

Reported-by: Simon Horman <simon.horman@netronome.com>
Fixes: 23f1eca6 ("ARM: dts: BCM5301X: Specify MDIO bus in the DT")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>