arm64: dts: imx8dxl: remove "fsl,imx7ulp-lpuart" compatible for imx8dxl

Drop the i.MX8DXL backwards compatibility with i.MX7ULP since they
differ from clocks handling point of view.

dtbs_check has the following warnings:
linux-next/arch/arm64/boot/dts/freescale/imx8dxl-evk.dtb: serial@5a060000: compatible: 'oneOf' conditional failed, one must be fixed:
        ['fsl,imx8qxp-lpuart', 'fsl,imx7ulp-lpuart'] is too long
        'fsl,imx93-lpuart' was expected
        'fsl,imx8qxp-lpuart' is not one of ['fsl,imx8qm-lpuart', 'fsl,imx8dxl-lpuart']
        'fsl,imxrt1050-lpuart' was expected
        'fsl,imx8ulp-lpuart' was expected
        'fsl,imx8qxp-lpuart' was expected
        'fsl,imxrt1170-lpuart' was expected
        From schema: linux-next/Documentation/devicetree/bindings/serial/fsl-lpuart.yaml

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>