dt-bindings: clock: Correct example in i.MX8QXP LPCG binding

In i.MX8QXP LPCG binding's example, "fsl,imx7d-usdhc" as fallback
compatible is incorrect, remove it to avoid below build error:

Documentation/devicetree/bindings/clock/imx8qxp-lpcg.example.dt.yaml:
mmc@5b010000: compatible: Additional items are not allowed ('fsl,imx7d-usdhc' was unexpected)
Documentation/devicetree/bindings/clock/imx8qxp-lpcg.example.dt.yaml:
mmc@5b010000: compatible: ['fsl,imx8qxp-usdhc', 'fsl,imx7d-usdhc'] is too long

Signed-off-by: Anson Huang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/clock/imx8qxp-lpcg.yaml

···
     };
 
     mmc@5b010000 {
-        compatible = "fsl,imx8qxp-usdhc", "fsl,imx7d-usdhc";
+        compatible = "fsl,imx8qxp-usdhc";
         interrupts = <GIC_SPI 232 IRQ_TYPE_LEVEL_HIGH>;
         reg = <0x5b010000 0x10000>;
         clocks = <&conn_lpcg IMX_CONN_LPCG_SDHC0_IPG_CLK>,