dt-bindings: serial: fsl-lpuart: Add imx8ulp compatible string

The lpuart on i.MX8ULP is derived from i.MX7ULP, it uses two compatible
strings, so update the compatible string for i.MX8ULP.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20211120113454.785997-3-peng.fan@oss.nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>