dt-bindings: serial: lpuart: add imx8qxp compatible string

Add imx8qxp compatible string

Cc: Mark Rutland <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Dong Aisheng <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
     on LS1021A SoC with 32-bit big-endian register organization
   - "fsl,imx7ulp-lpuart" for lpuart compatible with the one integrated
     on i.MX7ULP SoC with 32-bit little-endian register organization
+  - "fsl,imx8qxp-lpuart" for lpuart compatible with the one integrated
+    on i.MX8QXP SoC with 32-bit little-endian register organization
 - reg : Address and length of the register set for the device
 - interrupts : Should contain uart interrupt
 - clocks : phandle + clock specifier pairs, one for each entry in clock-names