ARM: dts: sun7i: Add ethernet alias for GMAC

All Allwinner A20 boards we support can only use either EMAC or GMAC,
as they share the same pins. As we have switched all supported to
GMAC, we should alias GMAC (the active controller) as ethernet0,
so u-boot will insert the MAC address for the correct controller.

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