ARM: dts: sun6i: Add GMAC clock node to the A31 dtsi

The GMAC uses 1 of 2 sources for its transmit clock, depending on the
PHY interface mode. Add both sources as dummy clocks, and as parents
to the GMAC clock node.

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