efi: sandbox: Adjust memory usage for sandbox

With sandbox the U-Boot code is not mapped into the sandbox memory range
so does not need to be excluded when allocating EFI memory. Update the EFI
memory init code to take account of that.

Signed-off-by: Simon Glass <sjg@chromium.org>
[agraf: Remove map_sysmem() call and header reference]
Signed-off-by: Alexander Graf <agraf@suse.de>