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>
diff --git a/lib/asm-offsets.c b/lib/asm-offsets.c
index b04f7c6..f4f1bb8 100644
--- a/lib/asm-offsets.c
+++ b/lib/asm-offsets.c
@@ -38,5 +38,7 @@
 
 	DEFINE(GD_START_ADDR_SP, offsetof(struct global_data, start_addr_sp));
 
+	DEFINE(GD_NEW_GD, offsetof(struct global_data, new_gd));
+
 	return 0;
 }