dt-bindings: net: dsa: document additional Microchip KSZ8863/8873 switch

It is a 3-Port 10/100 Ethernet Switch. One CPU-Port and two
Switch-Ports.

Cc: devicetree@vger.kernel.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>