dt-bindings: marvell: Add Marvell MV88E6060 DSA schema

The Marvell MV88E6060 is one of the oldest DSA switches from
Marvell, and it has DT bindings used in the wild. Let's define
them properly.

It is different enough from the rest of the MV88E6xxx switches
that it deserves its own binding.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>