gpio: Add TODO item for debugfs interface

The idea to create a debugfs to replace the aging and
dangerous sysfs ABI for hacking and tinkering came up
on the list.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201204083533.65830-1-linus.walleij@linaro.org