LF-1106 arm64: dts: imx8mq-evk: reduce cma reserved memory size to 896M

LF-1106 arm64: dts: imx8mq-evk: reduce cma reserved memory size to 896M

Currently the cma reserved memory allocation will fail due to conflict
with rpmsg reserved memory which results in a lot of VPU test failure.

e.g
rpmsg_reserved: rpmsg@0xb8000000 {
        no-map;
        reg = <0 0xb8000000 0 0x400000>;
};

This patch worked around this issue first by reducing the CMA size to 896M
from 960M.

We're still investigating a more sane approach to add back the correct
requirement CMA memory (per MM team, it may need 1060M).

Reviewed-by: ZHOU PENG <eagle.zhou@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>