ARM: dts: sun6i: Add pin muxing options for GMAC

The A31 SoC has a GMAC gigabit ethernet controller supporting
MII, GMII, RGMII modes. Add pin muxing options for these modes.

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