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

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 <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1592450578-30140-3-git-send-email-Anson.Huang@nxp.com
Signed-off-by: Rob Herring <robh@kernel.org>