board: rpi: Update memory layout for Aarch64

We are seeing the following issue booting when moving to the v6.8.x
kernel releases:

   Moving Image from 0x80000 to 0x200000, end=2600000
   ## Flattened Device Tree blob at 02600000
      Booting using the fdt blob at 0x2600000
   ERROR: FDT image overlaps OS image (OS=0x200000..0x2600000)

ARM and Aarch64 have different restrictions and trying to accommodate
larger kernels like the ones used in distros can be challenging. For this
reason, separate the layout and rearrange the map for Aarch64 to support
kernels larger than 36 MB.

The current approach is to reserve 128 MB for the kernel to allow boards
with 512 MB to have enough free space for other artifacts, such as
ramdisk.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>