dt-bindings: net: dsa: ksz: document Microchip KSZ87xx family switches

Document Microchip KSZ87xx family switches. These include
KSZ8765 - 5 port switch
KSZ8794 - 4 port switch
KSZ8795 - 5 port switch

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: David S. Miller <davem@davemloft.net>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Tristram Ha <Tristram.Ha@microchip.com>
Cc: Vivien Didelot <vivien.didelot@gmail.com>
Cc: Woojung Huh <woojung.huh@microchip.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>