arm64: dts: imx8: add mu5/6 node

Add mu5/6 for i.MX8QXP/QM, these two mu will be used for
communicating with general purpose Cortex-M4 cores.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>