gpiolib: remove leftover spinlock bits

We no longer use any spinlocks in gpiolib.c. Stop including
linux/spinlock.h and remove an outdated comment.

Link: https://lore.kernel.org/r/20241024191532.78304-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>