ARM: dts: sun7i: Add pin muxing options for the GMAC

The A20 has EMAC and GMAC muxed on the same pins.
Add pin sets with gmac function for MII and RGMII mode to the DTSI.

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