verdin-imx8mm: verdin-imx8mp: update env memory layout

Update the distro config env memory layout for the Verdin iMX8M Mini and
Verdin iMX8M Plus:

- loadaddr=0x48280000 allows for 128.5MB area for uncompressing (ie FIT
  images, kernel_comp_addr_r, kernel_comp_size)
- fdt_addr_r = loadaddr + 127.5MB : allows for 127.5MB kernel
- scriptaddr = fdt_addr_r + 512KB : allows for 512KB fdt
- ramdisk_addr_r = scriptaddr + 512KB : allows for 512KB script

Memory layout taken from commit fd5c7173
("imx8m{m,n}_venice: update env memory layout") but moved loadaddr by an
additional 0.5MB to avoid "Moving Image from 0x48200000 to 0x48280000"
during booti plus actually defining kernel_comp_size to make booti work.

Note that for our regular BSP Layers and Reference Images for Yocto
Project an updated distro boot script is required (see
meta-toradex-bsp-common/recipes-bsp/u-boot/u-boot-distro-boot).

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>