ARM: dts: BCM5301X: Specify MDIO bus in the DT

Northstar devices have MDIO bus that may contain various PHYs attached.
A common example is USB 3.0 PHY (that doesn't have an MDIO driver yet).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>