ARMv8: get new GD address from gd->new_gd directly

the new GD address is calculated via board data BD currently
it require the new GD area locates below BD tightly, so a strict
constraint is imposed on memory layout which maybe make special
platform unpleasant.

fix it by getting new GD address from gd->new_gd directly.

Signed-off-by: zijun_hu <zijun_hu@htc.com>
Reviewed-by: Simon Glass <sjg@chromium.org>