dt-bindings: mailbox: imx-mu: add imx7ulp MU support

There is a version 1.0 MU on imx7ulp, use "fsl,imx7ulp-mu" compatible
to support it.
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

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

@@ -21,6 +21,8 @@ Required properties:
 		imx6sx, imx7s, imx8qxp, imx8qm.
 		The "fsl,imx6sx-mu" compatible is seen as generic and should
 		be included together with SoC specific compatible.
+		There is a version 1.0 MU on imx7ulp, use "fsl,imx7ulp-mu"
+		compatible to support it.
 - reg :		Should contain the registers location and length
 - interrupts :	Interrupt number. The interrupt specifier format depends
 		on the interrupt controller parent.