dt-binding: gpio: publish binding IDs under dual license

Changes gpio.h DT binding header file to be published under GPLv2 or
BSD-2-Clause license terms. This change allows this GPIO generic
bindings header file to be used in software components as bootloaders
and OSes that are not published under GPLv2 terms.

All contributors to gpio.h file in copy.

Cc: Stephen Warren <swarren@nvidia.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Andrew Jeffery <andrew@aj.id.au>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220905145555.674800-1-etienne.carriere@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>