dt-bindings: net: Revert sun8i dwmac binding

This binding still doesn't please everyone, and we're getting far too
close from the release to allow it to reach a stable version.

Let's remove it until the discussion settles down.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>