Add device-tree binding for ar8xxx switch families.

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