gpio: restore original GPLv2+ license of gpiolib-of.c sources

It's easy to verify that the change of drivers/gpio/gpiolib-of.c license
header to SPDX standard changes the license from GPLv2+ to GPLv2, and
this change corrects it.

Fixes: dae5f0af

 ("gpio: Use SPDX header for core library")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>