imx8m: config: convert to bootm_size

Restrict the memory range available for image processing in the
"bootm" to 256 MiB so the kernel can access it and FDT or initrd are
not overwritten on ARM64.

Signed-off-by: Grygorii Tertychnyi <grygorii.tertychnyi@leica-geosystems.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Tom Rini <trini@konsulko.com>