ARM: dts: sun7i: Enable gmac on Wits Pro A20 DKT

The Wits Pro A20 DKT has a gbit ethernet port, enable it.

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