Revert "arm: bootm: Disable LMB reservation for command line and board info on arm64"

This reverts commit 2359fa7a.

While the goal is valid and there is surely unused memory in that area,
we also have a lot of crucial things still located at the top-of-memory
while running lmb_alloc_base. Such things are the page table (tlb_addr),
relocated U-Boot and the active stack. Possibly more. So this patch was
premature, we will need relocations of those things first if we want to
use the range.

Fixes booting on the IOT2050, but likely also on other boards. It got
stuck on relocating the FDT - over the relocated U-Boot code.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>