ARM: dts: r7s9210: Add USB clock

Add USB clock node. If present, this clock input must be 48MHz.
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/r7s9210.dtsi

@@ -30,6 +30,13 @@ rtc_x1_clk: rtc_x1 {
 		clock-frequency = <0>;
 	};
 
+	usb_x1_clk: usb_x1 {
+		#clock-cells = <0>;
+		compatible = "fixed-clock";
+		/* If clk present, value (48000000) must be set by board */
+		clock-frequency = <0>;
+	};
+
 	cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;