arm64: dts: imx8mn: Remove setting for IMX8MN_CLK_USB_CORE_REF

Since IMX8MN_CLK_USB_CORE_REF is not used at all, so remove the setting
for it.
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mn.dtsi

@@ -683,10 +683,8 @@ usbotg1: usb@32e40000 {
 				interrupts = <GIC_SPI 40 IRQ_TYPE_LEVEL_HIGH>;
 				clocks = <&clk IMX8MN_CLK_USB1_CTRL_ROOT>;
 				clock-names = "usb1_ctrl_root_clk";
-				assigned-clocks = <&clk IMX8MN_CLK_USB_BUS>,
-						  <&clk IMX8MN_CLK_USB_CORE_REF>;
-				assigned-clock-parents = <&clk IMX8MN_SYS_PLL2_500M>,
-							 <&clk IMX8MN_SYS_PLL1_100M>;
+				assigned-clocks = <&clk IMX8MN_CLK_USB_BUS>;
+				assigned-clock-parents = <&clk IMX8MN_SYS_PLL2_500M>;
 				fsl,usbphy = <&usbphynop1>;
 				fsl,usbmisc = <&usbmisc1 0>;
 				status = "disabled";