arm64: dts: imx8qxp: fix mbox-cells

Currently lsio_mu1 is used by Linux Kernel with mbox-cells as 2, but
actually mu0-4 could be used to communicate with SCU. So fix the
mbox-cells.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Fixes: 3d91ba65 ("arm64: dts: imx: add imx8qxp support")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>