Documentation: devicetree: add qca8k binding

Add device-tree binding for ar8xxx switch families.

Cc: devicetree@vger.kernel.org
Signed-off-by: John Crispin <john@phrozen.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>