gpio: Add global TODO file for GPIO

With a new subsystem co-maintainer on board it is good to draw
up the ongoing changes and future plans for the subsystem, i.e.
what is in my head and being worked on long term.

What better way is there than simply adding a TODO right in the
code and send it out to the mailing list.

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