gpio: Update TODO

Drop the completed item: hierarchical irqchip helpers. Add
motivation for gpio descriptor refactoring. Extend the list of
stuff to do. Minor fixups.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200107212432.27587-1-linus.walleij@linaro.org