arm64: dts: imx8dxl-ss-conn: Move clk_dummy out of USB node

The clk_dummy is not part of the usbotg2 block, so move it
to the root node to fix the following dt-schema warning:

imx8dxl-evk.dtb: usb@5b0e0000: Unevaluated properties are not allowed ('clock-dummy' was unexpected)
	from schema $id: http://devicetree.org/schemas/usb/ci-hdrc-usb2.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8dxl-ss-conn.dtsi

@@ -13,6 +13,13 @@ conn_enet0_root_clk: clock-conn-enet0-root {
 		clock-frequency = <250000000>;
 		clock-output-names = "conn_enet0_root_clk";
 	};
+
+	clk_dummy: clock-dummy {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+		clock-frequency = <0>;
+		clock-output-names = "clk_dummy";
+	};
 };
 
 &conn_subsys {
@@ -53,13 +60,6 @@ usbotg2: usb@5b0e0000 {
 		rx-burst-size-dword = <0x10>;
 		power-domains = <&pd IMX_SC_R_USB_1>;
 		status = "disabled";
-
-		clk_dummy: clock-dummy {
-			compatible = "fixed-clock";
-			#clock-cells = <0>;
-			clock-frequency = <0>;
-			clock-output-names = "clk_dummy";
-		};
 	};
 
 	usbmisc2: usbmisc@5b0e0200 {