ARM: dts: sun6i: Add A31 GMAC gigabit ethernet controller node

The A31 has the same GMAC found on the A20 SoC, except it has
an extra reset control.

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