net: dsa: add new DSA switch driver for the SMSC-LAN9303

The SMSC/Microchip LAN9303 is an ethernet switch device with one CPU port
and two external ethernet ports with built-in phys.

This driver uses the DSA framework, but is currently only capable of
separating the two external ports. There is no offload support yet.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>