imx8m{m,n}_venice: update env memory layout

Update distro config env memory layout:
 - loadaddr=0x48200000 allows for 130MB area for uncompressing (ie FIT
   images, kernel_comp_addr_r)
 - fdt_addr_r = loadaddr + 128MB - allows for 128MB kernel
 - scriptaddr = fdt_addr_r + 512KB - allows for 512KB fdt
 - ramdisk_addr_r = scriptaddr + 512KB - allows for 512KB script

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>