gpio: Add TODO item for regmap helper

Add a TODO item for a generic MMIO regmap helper.

Suggested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>