dt-bindings: serial: lpuart: add imx8qm compatible string

Add imx8qm compatible string.
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -10,6 +10,8 @@ Required properties:
     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
+  - "fsl,imx8qm-lpuart" for lpuart compatible with the one integrated
+    on i.MX8QM 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