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>

arch/arm/boot/dts/sun9i-a80.dtsi

@@ -183,6 +183,37 @@ osc32k: clk-32k {
 			clock-output-names = "osc32k";
 		};
 
+		/*
+		 * The following two are dummy clocks, placeholders
+		 * used in the gmac_tx clock. The gmac driver will
+		 * choose one parent depending on the PHY interface
+		 * mode, using clk_set_rate auto-reparenting.
+		 *
+		 * The actual TX clock rate is not controlled by the
+		 * gmac_tx clock.
+		 */
+		mii_phy_tx_clk: mii_phy_tx_clk {
+			#clock-cells = <0>;
+			compatible = "fixed-clock";
+			clock-frequency = <25000000>;
+			clock-output-names = "mii_phy_tx";
+		};
+
+		gmac_int_tx_clk: gmac_int_tx_clk {
+			#clock-cells = <0>;
+			compatible = "fixed-clock";
+			clock-frequency = <125000000>;
+			clock-output-names = "gmac_int_tx";
+		};
+
+		gmac_tx_clk: clk@800030 {
+			#clock-cells = <0>;
+			compatible = "allwinner,sun7i-a20-gmac-clk";
+			reg = <0x00800030 0x4>;
+			clocks = <&mii_phy_tx_clk>, <&gmac_int_tx_clk>;
+			clock-output-names = "gmac_tx";
+		};
+
 		cpus_clk: clk@8001410 {
 			compatible = "allwinner,sun9i-a80-cpus-clk";
 			reg = <0x08001410 0x4>;