dt-bindings: net-next: Add DT bindings documentation for Allwinner dwmac-sun8i

This patch adds documentation for Device-Tree bindings for the
Allwinner dwmac-sun8i driver.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>