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>

arch/arm/lib/crt0_64.S

@@ -95,8 +95,7 @@ ENTRY(_main)
  */
 	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 */

lib/asm-offsets.c

@@ -38,5 +38,7 @@ int main(void)
 
 	DEFINE(GD_START_ADDR_SP, offsetof(struct global_data, start_addr_sp));
 
+	DEFINE(GD_NEW_GD, offsetof(struct global_data, new_gd));
+
 	return 0;
 }