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>