ARM: dts: sun7i: Add the muxing options for the EMAC

The A20 has several muxing options for the EMAC. Yet, the currently
supported boards only use one set of them. Add that pin set to the DTSI.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>