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

The MDIO node on Cygnus had an reversed #address-cells and
 #size-cells properties, correct those.

Fixes: 40c26d3a ("ARM: dts: Cygnus: Add the ethernet switch and ethernet PHY")
Reported-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>