net: dsa: realtek: get internal MDIO node by name

The binding docs requires for SMI-connected devices that the switch
must have a child node named "mdio" and with a compatible string of
"realtek,smi-mdio". Meanwile, for MDIO-connected switches, the binding
docs only requires a child node named "mdio".

This patch changes the driver to use the common denominator for both
interfaces, looking for the MDIO node by name, ignoring the compatible
string.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>