net: dsa: Add DT bindings for Vitesse VSC73xx switches

This adds the device tree bindings for the Vitesse VSC73xx
switches. We also add the vendor name for Vitesse.

Cc: devicetree@vger.kernel.org
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>