efi_loader: Put fdt into convenient location

The uEFI spec doesn't dictate where the device tree should live at, but
legacy 32bit ARM grub2 has some assumptions that it may stay at its place
when it's already loaded by the firmware.

So let's put it somewhere where Linux that comes after would happily find
it - around the recommended 128MB line.

Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>