net: dsa: LAN9303: add I2C managed mode support

In this mode the switch device and the internal phys will be managed via
I2C interface. The MDIO interface is still supported, but for the
(emulated) CPU port only.

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