ARM: dts: sun9i: Add A80 GMAC RGMII pinmux setting

The GMAC (gigabit ethernet controller) supports RGMII to connect to
the ethernet PHY, for gigabit network speeds.

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