arm64: dts: rockchip: Add gmac_clkin fixed clock for gmac usage for px30

Change-Id: Ibb155666235c609603e0a89316398c3aeb303805
Signed-off-by: David Wu <david.wu@rock-chips.com>

arch/arm64/boot/dts/rockchip/px30.dtsi

@@ -133,6 +133,13 @@
 		};
 	};
 
+	gmac_clkin: external-gmac-clock {
+		compatible = "fixed-clock";
+		clock-frequency = <50000000>;
+		clock-output-names = "gmac_clkin";
+		#clock-cells = <0>;
+	};
+
 	psci {
 		compatible = "arm,psci-1.0";
 		method = "smc";