dt-bindings: mailbox: imx-mu: correct example

The example use i.MX8QXP MU, but actually the MU is compatible with
i.MX6SX, so add the compatible.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.txt

@@ -51,7 +51,7 @@ Optional properties:
 Examples:
 --------
 lsio_mu0: mailbox@5d1b0000 {
-	compatible = "fsl,imx8qxp-mu";
+	compatible = "fsl,imx8qxp-mu", "fsl,imx6sx-mu";
 	reg = <0x0 0x5d1b0000 0x0 0x10000>;
 	interrupts = <GIC_SPI 176 IRQ_TYPE_LEVEL_HIGH>;
 	#mbox-cells = <2>;