x86: Add a handoff header file

Add an arch-specific handoff header so that we can use the HANDOFF feature
on x86 devices.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/handoff.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Architecture-specific SPL handoff information for x86
+ *
+ * Copyright 2018 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ */
+
+#ifndef __x86_asm_handoff_h
+#define __x86_asm_handoff_h
+
+struct arch_spl_handoff {
+};
+
+#endif