dt-bindings: net: mvusb: Fix up DSA example

When adding a proper schema for the Marvell mx88e6xxx switch,
the scripts start complaining about this embedded example:

dtschema/dtc warnings/errors:
net/marvell,mvusb.example.dtb: switch@0: ports: '#address-cells'
is a required property
from schema $id: http://devicetree.org/schemas/net/dsa/marvell,mv88e6xxx.yaml#
net/marvell,mvusb.example.dtb: switch@0: ports: '#size-cells'
is a required property
from schema $id: http://devicetree.org/schemas/net/dsa/marvell,mv88e6xxx.yaml#

Fix this up by extending the example with those properties in
the ports node.

While we are at it, rename "ports" to "ethernet-ports" and rename
"switch" to "ethernet-switch" as this is recommended practice.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231127-marvell-88e6152-wan-led-v9-2-272934e04681@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>