gpio: generic: move GPIO_GENERIC_ flags to the correct header
These flags are specific to gpio-mmio and belong in linux/gpio/generic.h
so move them there.
Link: https://lore.kernel.org/r/20250917-gpio-generic-flags-v1-2-69f51fee8c89@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>