ARM: dts: sun9i: Add GMAC clock node

The A80 has the same DWMAC hardware as on earlier Allwinner SoCs. The
accompanying GMAC clock register has been moved into the "System
Control" area.

Add a clock node for it.

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