platform/x86: intel_chtdc_ti_pwrbtn: Add SPDX identifier

Driver misses the licence text or identifier, thus, append it here.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/intel_chtdc_ti_pwrbtn.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Power-button driver for Dollar Cove TI PMIC
  * Copyright (C) 2014 Intel Corp