ARM: dts: sun9i: Add A80 GMAC gigabit ethernet controller node

The A80 has the same GMAC found on the A31 SoC.

Add a device node, and an alias for it.

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