ARM: dts: imx7s: add i.MX7 messaging unit support

Define the Messaging Unit (MU) for i.MX7 in the processor's dtsi.
The respective driver is added in the next commit.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>