dt-bindings: mfd: Dual licensing for st,stpmic1 bindings

Change include/dt-bindings/mfd/st,stpmic1.h license model from GPLv2.0
only to dual GPLv2.0 or BSD-2-Clause. I have every legitimacy to request
this change on behalf of STMicroelectronics. This change clarifies that
this DT binding header file can be shared with software components as
bootloaders and OSes that are not published under GPLv2 terms.

In CC are all the contributors to this header file.

Cc: Pascal Paillet <p.paillet@st.com>
Cc: Lee Jones <lee@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240617092016.2958046-1-etienne.carriere@foss.st.comSigned-off-by: Lee Jones <lee@kernel.org>

include/dt-bindings/mfd/st,stpmic1.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
 /*
  * Copyright (C) STMicroelectronics 2018 - All Rights Reserved
  * Author: Philippe Peurichard <philippe.peurichard@st.com>,