LF-3767 imx8m: set mem=xMB to bootargs

LF-3767 imx8m: set mem=xMB to bootargs

Below two commit in Kernel cause no-map reserved memory are still kept
in kernel as memblock.
54d3a5761951 of/fdt: Make sure no-map does not remove already reserved regions
0674fa99a7d8 fdt: Properly handle "no-map" field in the memory region

So when jailhouse enabling, there will be resource conflict because
jailhouse pci will request the reserved memory for itself handling
virtual PCI trap.

So let's cut off the reserved memory using mem=xMB to avoid such issue.

Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>