ARM: dts: owl-s500-guitar-bb-rev-b: Add fake uart3 clock

Add a fixed-clock for baudrate 115200.
Signed-off-by: NAndreas Färber <afaerber@suse.de>

arch/arm/boot/dts/owl-s500-guitar-bb-rev-b.dts

@@ -19,8 +19,15 @@
 	chosen {
 		stdout-path = "serial3:115200n8";
 	};
+
+	uart3_clk: uart3-clk {
+		compatible = "fixed-clock";
+		clock-frequency = <921600>;
+		#clock-cells = <0>;
+	};
 };
 
 &uart3 {
 	status = "okay";
+	clocks = <&uart3_clk>;
 };