dt-bindings: serial: fsl-lpuart: correct imx93-lpuart dt-binding item

Correct the fsl,imx93-lpuart dt-binding item, imx93/imx8ulp add some new
features based on imx7ulp lpuart, so need to add "fsl,imx8ulp-lpuart"
for imx93 to enable those new features.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230705015602.29569-4-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>