efi: Tidy up device-tree-size calculation in copy_fdt()

This is a bit confusing at present since it adds 4KB to the pointer, then
rounds it up. It looks like a bug, but is not.

Move the 4KB addition into a separate statement and expand the comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>