iio: adc: Update QCOM ADC license to SPDX format

Update QCOM ADC header file to SPDX format.
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

include/dt-bindings/iio/qcom,spmi-vadc.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (c) 2012-2014,2018 The Linux Foundation. All rights reserved.
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 and
- * only version 2 as published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
  */
 
 #ifndef _DT_BINDINGS_QCOM_SPMI_VADC_H