gpio: TODO: add a task for removing MMIO-specific fields from gpio_chip

gpio: TODO: add a task for removing MMIO-specific fields from gpio_chip

Currently for CONFIG_GPIO_GENERIC=y each struct gpio_chip object
contains the fields relevant only for gpio-mmio users. It's not an
insignificant number either as it's several pointers and integers.

It makes sense to remove these fields from struct gpio_chip into a
dedicated structure but this is not trivial due to how the bgpio_init()
function is implemented.

Add a task for tracking this rework.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250527111353.71540-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>