gpio: Add GPIOLIB_IRQCHIP cleanup to TODO

We now have two APIs for registering GPIOLIB_IRQCHIP, this is not
working and creating confusion. Add a TODO item to fix it up.

Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>