cpu/intel/hyperthreading: Add missing header <arch/cpu.h>

This file is using cpuid_result and cpuid(). I also removed the spinlock
header since it's not used. This is what was previously providing the
cpu.h header.

BUG=b:179699789
TEST=none

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Idc3daa64562c4a4d57b678f13726509b480ba050
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/cpu/intel/hyperthreading/intel_sibling.c

@@ -1,10 +1,10 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <arch/cpu.h>
 #include <console/console.h>
 #include <cpu/intel/hyperthreading.h>
 #include <device/device.h>
 #include <option.h>
-#include <smp/spinlock.h>
 
 /* Intel hyper-threading requires serialized CPU init. */