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/arch/arm/lib/crt0_64.S b/arch/arm/lib/crt0_64.S
index 62fad45..9c46c93 100644
--- a/arch/arm/lib/crt0_64.S
+++ b/arch/arm/lib/crt0_64.S
@@ -95,8 +95,7 @@
  */
 	ldr	x0, [x18, #GD_START_ADDR_SP]	/* x0 <- gd->start_addr_sp */
 	bic	sp, x0, #0xf	/* 16-byte alignment for ABI compliance */
-	ldr	x18, [x18, #GD_BD]		/* x18 <- gd->bd */
-	sub	x18, x18, #GD_SIZE		/* new GD is below bd */
+	ldr	x18, [x18, #GD_NEW_GD]		/* x18 <- gd->new_gd */
 
 	adr	lr, relocation_return
 	ldr	x9, [x18, #GD_RELOC_OFF]	/* x9 <- gd->reloc_off */