dt-bindings: dsa: Rewrite Vitesse VSC73xx in schema

This rewrites the Vitesse VSC73xx DSA switches DT binding in
schema.

It was a bit tricky since I needed to come up with some way
of applying the SPI properties only on SPI devices and not
platform devices, but I figured something out that works.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>