dt-bindings: net: marvell,dfx-server: Convert to yaml

Even though this description is not used anywhere upstream (no matching
driver), while on this file I decided I would try a conversion to yaml
in order to clarify the prestera family description.

I cannot keep the nodename dfx-server@xxxx so I switched to dfx-bus@xxxx
which matches simple-bus.yaml. Otherwise I took the example context from
the only user of this compatible: armada-xp-98dx3236.dtsi, which is a
rather old and not perfect DT.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>