ARM: dts: sun7i: cubieboard2: Enable GMAC instead of EMAC

GMAC has better performance and fewer hardware issues.
Use the GMAC in MII mode for ethernet instead of the EMAC.

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