net: dsa: Add Vitesse VSC73xx DSA router driver

This adds a DSA driver for:

Vitesse VSC7385 SparX-G5 5-port Integrated Gigabit Ethernet Switch
Vitesse VSC7388 SparX-G8 8-port Integrated Gigabit Ethernet Switch
Vitesse VSC7395 SparX-G5e 5+1-port Integrated Gigabit Ethernet Switch
Vitesse VSC7398 SparX-G8e 8-port Integrated Gigabit Ethernet Switch

These switches have a built-in 8051 CPU and can download and execute
firmware in this CPU. They can also be configured to use an external
CPU handling the switch in a memory-mapped manner by connecting to
that external CPU's memory bus.

This driver (currently) only takes control of the switch chip over
SPI and configures it to route packages around when connected to a
CPU port. The chip has embedded PHYs and VLAN support so we model it
using DSA as a best fit so we can easily add VLAN support and maybe
later also exploit the internal frame header to get more direct
control over the switch.

The four built-in GPIO lines are exposed using a standard GPIO chip.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>