dt-bindings: pinctrl: stm32: fix copyright and adopt SPDX identifier

Add missing copyright and add SPDX identifier.
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/dt-bindings/pinctrl/stm32-pinfunc.h

+/* SPDX-License-Identifier: (GPL-2.0+ OR BSD-3-Clause) */
+/*
+ * Copyright (C) STMicroelectronics 2017 - All Rights Reserved
+ * Author: Torgue Alexandre <alexandre.torgue@st.com> for STMicroelectronics.
+ */
+
 #ifndef _DT_BINDINGS_STM32_PINFUNC_H
 #define _DT_BINDINGS_STM32_PINFUNC_H