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>