net: dsa: add SMSC/Microchip LAN9303 three port ethernet switch driver

[Juergen Borleis]

The LAN9303 is a three port 10/100 ethernet switch with integrated phys
for the two external ethernet ports. The third port is an RMII/MII
interface to a host master network interface (e.g. fixed link).

While the LAN9303 device itself supports offload packet processing, this
driver does not make use of it yet. This driver just configures the device
to provide two separate network interfaces (which is the default state of
a DSA device).

Please note: the "MDIO managed mode" driver part isn't tested yet. I have
used and tested the "I2C managed mode" only.

Comments are welcome.

jb