arm64: use store with auto-increment

Save one instruction.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/lib/crt0_64.S

@@ -118,8 +118,7 @@ relocation_return:
 	ldr	x0, =__bss_start		/* this is auto-relocated! */
 	ldr	x1, =__bss_end			/* this is auto-relocated! */
 clear_loop:
-	str	xzr, [x0]
-	add	x0, x0, #8
+	str	xzr, [x0], #8
 	cmp	x0, x1
 	b.lo	clear_loop