cmd: bootefi: allocate device-tree copy from high memory

The bootefi command creates a copy of the device-tree within the first
127 MiB of memory. This may lead to overwriting previously loaded binaries
(e.g. kernel, initrd).

Linux EFI stub itself copies U-Boot's copy of the device-tree. This means
there is not restriction for U-Boot to place the device-tree copy to any
address. (Restrictions existed for 32bit ARM before Linux commit
7a1be318f579 ("ARM: 9012/1: move device tree mapping out of linear region")
for legacy booting.

Reported-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>