net: dsa: LAN9303: add MDIO managed mode support

When the LAN9303 device is in MDIO manged mode, all register accesses must
be done via MDIO.

Please note: this code is compile time tested only due to the absence of such
configured hardware. It is based on a patch from Stefan Roese from 2014.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
CC: devicetree@vger.kernel.org
CC: robh+dt@kernel.org
CC: mark.rutland@arm.com
CC: sr@denx.de
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>