gpio: TODO: add item about GPIO drivers reading struct gpio_chip::base

gpio: TODO: add item about GPIO drivers reading struct gpio_chip::base

drivers/pinctrl/pinctrl-at91.c uses struct gpio_chip::base to find out
which bit to set in a register:

  dev_dbg(npct->dev, "enable pin %u as GPIO\n", offset);

  mask = 1 << (offset - chip->base);

This adds a non-obvious dependency on the global numberspace from the
GPIO driver itself, even if all consumers use the descriptor API.

More such instances may exist and will need to be fixed in the quest for
removal of the global numberspace, so add a reminder about it to the
TODO list.

Link: https://lore.kernel.org/all/1d00c056-3d61-4c22-bedd-3bae0bf1ddc4@pengutronix.de/
Suggested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20250507-gpio-chip-base-readback-v1-1-ade56e38480b@pengutronix.de
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>