dt-bindings: serial: fsl-lpuart: add i.MXRT1050 compatible

Add i.MXRT1050 documentation for compatible string.

Cc: Giulio Benetti <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jesse Taube <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-lpuart.yaml

···
           - fsl,ls1028a-lpuart
           - fsl,imx7ulp-lpuart
           - fsl,imx8qm-lpuart
+          - fsl,imxrt1050-lpuart
       - items:
           - enum:
               - fsl,imx8qxp-lpuart