rpi: Fix fdt_high & initrd_high for 64-bit builds

The magic value that disables relocation is dependent on the CPU word
size, so the current 'ffffffff' is doing the wrong thing on aarch64.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Signed-off-by: Alexander Graf <agraf@suse.de>