dt-bindings: net: add qca,ar71xx.txt documentation

Add binding documentation for Atheros/QCA networking IP core used
in many routers.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>